Optical Damage Resistance Research Articles

Investigation of Increased Photorefractive Damage Resistance in LiNbO3 by Two-Wave Mixing Measurements

The increase in photorefractive damage resistance in both MgO-doped and nondoped Li-deficient LiNbO3 is studied by two wave mixing measurements. Holographic diffraction measurements demonstrated that crystals with high optical damage resistance increase photoconductivity and response time for erasing photorefractive grating. The erasure speed is linearly proportional to the MgO dopant and Li-site vacancy concentrations. The erasure time of holographic grating shows an Arrhenius dependence on temperature from which activation energy is obtained. We proposed an energy level model of the light-induced transport for LiNbO3:MgO dopants introduce donor levels 0.11 eV below the conduction band, whereas, Li-site vacancies in nondoped crystal introduce acceptor levels 0.29–0.44 eV above the valence band.

Defect Structure Model of MgO-Doped LiNbO 3

To study the defect structure of MgO-doped lithium niobate, single crystals of lithium niobates (LiNbO 3, "LN") with varying MgO content were characterized by chemical analysis, lattice parameters, and density measurements. An Mg-incorporation mechanism was assumed on the basis of the chemical formulae derived from the data and in light of our recently proposed defect model of nondoped LN. At first, Mg would replace the Nb ion at the Li site and complete replacement would take place at 3% MgO doping keeping the molar ratio Li/Nb = 0.94. This corresponds to the formula [Li 0.94Mg 0.03□ 0.03] [Nb 1.0]O 3. Further Mg ions are incorporated into the Li site, replacing Li ions, with accompanying vacancy creation, down to Li/Nb = 0.84, which corresponds to the Nb-rich side limit of the LN solid solution range. The number of vacancies would reach a maximum at this composition and the formula would be [Li 0.84Mg 0.08□ 0.08] [Nb 1.0]O 3. Beyond this point, Mg ions enter the Nb and Li sites simultaneously, maintaining the Li/Nb ratio, leading to a decrease in vacancies. Two thresholds in the change of composition and properties reported so far in the literature can be interpreted by this model. Improved optical damage resistance due to MgO-doping was attributed to the increase in vacancies, and not by its decrease as was generally supposed.

Increased Optical Damage Resistance and Transparency in MgO-Doped LiTaO3 Single Crystals

Clear and transparent 2-inch diameter undoped and MgO-doped LiTaO3 single crystals with few Fe impurities have been grown by the conventional Czochralski method from a congruent melt. The optical damage resistance of undoped LiTaO3 has been characterized by the measurement of photoinduced birefringence change and is lower than that of commercially available LiNbO3, which is not consistent with the previously reported data in the literature. In addition, doping of a few mol% MgO further improved the optical damage resistance, transmission in the visible light region, and crystal decolorization, and produced a favorable shift of absorption edge toward shorter wavelengths for lithium tantalate. These are important advantages when considering shorter- wavelength accessibility and high conversion efficiency in second-harmonic- generation devices.

Increased optical damage resistance in Sc2O3-doped LiNbO3

The optical damage of 1 mol % Sc2O3-doped LiNbO3 was approximately two times smaller than an optical grade LiNbO3 measured as a function of Ar+ (λ=488 nm) irradiation time. Severe Ar+ beam distortion observed in the undoped LiNbO3 was not present in the Sc2O3-doped LiNbO3. There was negligible shift in the OH− absorption band but a 10 nm blue shift was observed in the absorption edge, indicating that Sc3+ and Mg2+ incorporation may proceed by a different mechanism. This is the first of reported results, to the authors’ knowledge, of a trivalent dopant increasing the damage resistance level in LiNbO3.

Optical damage resistance and crystal quality of LiNbO3 single crystals with various [Li]/[Nb] ratios

Lithium niobate single crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 45.5–58 mol % Li2O. Their optical damage resistance was characterized by measurement of the photoinduced birefringence change. Their crystal quality was characterized by x-ray topography and x-ray rocking curves. The photoinduced birefringence change increased with an increase in the Li content of the crystal. The Li-poor crystals, which were richer in Nb content than the crystal with the congruent composition, showed the smallest birefringence change, in spite of the fact that the x-ray rocking curve width of these Li-poor crystals was much wider than the congruent crystal. It is concluded that the optical damage resistance of undoped LiNbO3 is increased with an increase in the Nb content of the crystal, and that the crystal quality, as represented by x-ray rocking curve peak width, has no correlation with the optical damage resistance.

A new approach in the design of polar crystals for quadratic nonlinear optics exemplified by the synthesis and crystal structure of 2-amino-5-nitropyridinium dihydrogen monophosphate (2A5NPDP)

2-amino-5-nitropyridinium dihydrogen monophosphate exemplifies a new crystal engineering strategy combining mineral and organic moieties towards enhanced quadratic nonlinear optical properties. This strategy is meant to combine the advantages of mineral ionic structures (cohesion, stability, optical and other damage resistance) with those of organic molecules (structural flexibility, high optical polarizability). This organic inorganic material is designed so as to favour mutual dipolar interactions between the 2-amino-5-nitropyridinium cations and the (H2PO4−)n subnetwork interlocked by hydrogen bonding in the same crystalline lattice. This approach rests on the structural features and chemical properties of (H2PO4−)n polyanion. The 2A5NPDP structure reveals a polar layered arrangement. The high powder SHG efficiency of 2A5NPDP, comparable to that of state-of-the-art purely organic 3-methyl-4-nitropyridine-1-oxide (POM) crystals, confirms the validity of this approach, currently generalized to a large variety of mixed organo-mineral crystals.

Waveguides in Lithium Niobate

Lithium niobate (LiNbO3 is considered to be the leading electrooptical material for fabrication of active waveguides, modulators, and switches for application in integrated optical circuits. LiNbO3 is a ferroelectric material with a high Curie temperature of 1210°C. This is essential to permit rapid diffusion at high temperature, e.g., >1000°C, without domain reversal. Its high electrooptic coefficient allows modulation at relatively low voltages. Waveguides are fabricated in LiNbO3 by Ti indiffusion around 1000°C, by Li2O outdiffusion, and, more recently, by Li+/H+ ion exchange. We report results of a study on diluted Li+/H+ ion exchange. This process is attractive because it allows control of the index change between 0.003 and 0.1 Furthermore, the fabricated waveguides are polarization selective, have high optical damage resistance, and do not exhibit index instability as is the case with complete Li+/H+ exchange. The variation in both the index of refraction and the diffusion coefficient with composition is very nonlinear. Mechanisms contributing to these nonlinear properties are discussed.

Measurement of optical losses and damage resistance of ZnSNa 3AlF 6 and TiO 2SiO 2 laser mirrors depending on coating design

Quarterwave ZnSNa 3AlF 6 and TiO 2SiO 2 multilayer stacks were prepared to investigate correlations between optical absorption, scattering and laser damage resistance at λ = 488 nm, 515 nm and 530 nm respectively. The coating design was varied to assist the investigation. The ratio of interface to volume contributions to the optical losses was shown to depend on the coating material and deposition process used. The highest damage resistance was found in multilayer systems which consist of an alternating set-up of a λ/4 high refracting and a 3λ/4 low refracting component. Thus, the correlation between absorption and damage may not be established in each case, as the damage becomes design dependent. To explain this result, it is necessary to take into account the heat flow within the stacks investigated during laser irradiation as well as an additionally acting laser-induced thermal stress giving rise to a different damage resistance in the multilayer stacks.

Optical coatings deposited using ion assisted deposition

The properties of Ta2O5 and Al2O3 optical coatings deposited using oxygen-ion assisted deposition (IAD) were investigated. Previously,1 we reported preliminary results for Ta2O5 and Al2O3 coatings deposited using IAD. In this paper, we present results illustrating the effects of oxygen-ion bombardment on film optical constants, environmental durability, and laser damage resistance. The coatings were bombarded with 200-, 300-, 500-, and 1000-eV oxygen ions during deposition. Increased values of refractive index were obtained for coatings deposited with simultaneous O+2 bombardment. Antireflection coatings deposited using IAD did not exhibit increased laser damage thresholds at λ=351 nm.

Electro-optic, linear, and nonlinear optical properties of KDP and its isomorphs

Abstract The electro-optic, linear, and nonlinear optical properties of KDP and its isomorphs are reviewed. Figures of merit for doubling and tripling by sum-frequency generation are presented and used to compare and contrast these crystals for nonlinear optics devices. Current research areas, such as high-average-power applications, and optical damage resistance are reviewed.

Optical damage resistance of ion-implanted LiNbO 3 waveguides

The letter reports the first data for optical damage studies of ion-implanted planar waveguides in LiNbO3. No optical damage was detected at visible laser wavelengths using laser sources of up to 5 W. In a comparison of ion-implanted and Ti-diffused waveguides the optical damage threshold for the ion-implanted guides is at least 100 times greater.

Increased optical damage resistance in lithium niobate

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.

Optical Damage Resistance Of Lithium Niobate Waveguides

The optical damage resistance of diffused lithium niobate wave-guides is discussed. Long-term laser power handling performance is assessed by recording the spatial intensity changes occurring as a function of time in a waveguide's far field. Out-diffused waveguides are found to offer significantly greater long-term resistance to optical power losses caused by optical damage than do waveguides formed by either ion exchange or titanium in-diffusion. Any waveguide's resistance to optical damage is lowered by inadvertent exposure to chemical reducing conditions that can prevail during device fabrication processes. Waveguides exposed to the severe chemical reducing conditions are found to exhibit dramatically nonlinear behavior when exposed to high laser powers and short laser wavelengths.

Microstructure of Evaporated Dielectric Films

Dielectric films are used as optical coatings in a variety of mirror and lens based optical systems. These materials are generally thin films formed by either evaporation or sputter deposition whose optical properties, perfection and damage resistance are influenced by changes in the deposition process. The present study is aimed at understanding the microstructural and microchemical state of several types of dielectric films and correlation of that state with the observed properties. Thin films of ZnS, ZnSe and ThF4 prepared in either of two different fashions have been examined: (1) free-standing films deposited on alkali halides and removed by dissolving the substrate, or (2) films deposited on thin silicon single crystals and then ion milled (from the Si side) to electron transparency. Conventional transmission electron microscopy was performed on a JEOL 120C microscope.

Optical damage resistance of monovalent ion diffused LiNbO3 and LiTaO3 waveguides

We report measurements of the zero field photorefractive sensitivity (optical damage) of waveguides fabricated by silver ion exchange in both LiNbO3 and LiTaO3 and of LiNbO3 :Ti guides diffused in the presence of H20. Marked differences in the photorefractive sensitivity are found for the different fabrication proceseses. In particular, LiNbO3 :Ti guides diffused in the presence of H20 were significantly less susceptible to optical damage than those diffused in a dry atmosphere, and no index changes were observed in LiTaO3 :Ag waveguides for exposures up to 105 J/cm2 at 4579 Å.