Dieter H. Jundt

Quasi-phase-matched second harmonic generation: tuning and tolerances

The theory of quasi-phase-matched second-harmonic generation is presented in both the space domain and the wave vector mismatch domain. Departures from ideal quasi-phase matching in periodicity, wavelength, angle of propagation, and temperature are examined to determine the tuning properties and acceptance bandwidths for second-harmonic generation in periodic structures. Numerical examples are tabulated for periodically poled lithium niobate. Various types of errors in the periodicity of these structures are then analyzed to find their effects on the conversion efficiency and on the shape of the tuning curve. This analysis is useful for establishing fabrication tolerances for practical quasi-phase-matched devices. A method of designing structures having desired phase-matching tuning curve shapes is also described. The method makes use of varying domain lengths to establish a varying effective nonlinear coefficient along the interaction length. >

Infrared corrected Sellmeier coefficients for congruently grown lithium niobate and 5 mol. % magnesium oxide–doped lithium niobate

The growth in the uses of lithium niobate for infrared applications has created a need for knowledge of its optical characteristics in the infrared spectral region for the purpose of designing phase-matched or quasi-phase-matched devices. We report measurements of the refractive indices of congruently grown lithium niobate and lithium niobate doped with 5 mol. % magnesium oxide. We use these results to predict the tuning curve of a room-temperature multigrating optical parametric oscillator in each material.

Preparation and characterization of off-congruent lithium niobate crystals

Vapor transport equilibration was used to prepare lithium niobate crystals of a variety of controlled off‐congruent compositions. Crystals were characterized through measurement of the ferroelectric Curie temperature Tc and measurement of the temperature for noncritical phase matching TPM of second‐harmonic generation from both 1.064‐ and 1.32‐μm laser sources. Across the majority of the single‐phase region, both Tc and TPM were observed to vary nearly linearly with Li/Nb ratio. The variation in TPM with Li/Nb ratio was observed to change direction within the single‐phase region at some small but finite compositional interval away from the Li‐rich phase boundary. Crystals equilibrated to the Li‐rich phase boundary had excellent optical homogeneity. Preparation of Li‐poor crystals was hampered by extremely slow lithium diffusivity and problems with second‐phase precipitation.

Optical properties of lithium-rich lithium niobate fabricated by vapor transport equilibration

Lithium-rich lithium niobate of excellent optical homogeneity can be fabricated by a vapor transport equilibration (VTE) technique. The high-optical-quality, uniformly birefringent crystals noncritically phase match for second-harmonic generation of 532-nm radiation from 1064-nm Nd:YAG radiation at 238 degrees C. The refractive indexes and their temperature dependence have been measured and used to derive temperature-dependent Sellmeier equations, which predict noncritical phase matching for wavelengths as short as 976 nm to generate blue at 488 nm at room temperature. The Sellmeier equations accurately predict experimental phase-matching temperatures over a wide temperature range. >

Periodically poled LiNbO3 for high‐efficiency second‐harmonic generation

Quasi‐phase‐matched room‐temperature frequency doubling to generate blue, green, and red light was demonstrated using periodically poled LiNbO3 crystals. A 1.24‐mm‐long sample in an external resonant cavity generated 1.7 W of green power from an input of 4.2 W at the 1.064 μm Nd:YAG laser line when heated to 140 °C to compensate for a slight error in periodicity.

Characterization of single‐crystal sapphire fibers for optical power delivery systems

Sapphire single‐crystal fibers with diameters of 110 μm and lengths of over 2 m have been grown by the laser‐heated pedestal growth method. The fibers are free of imperfections such as voids and bubbles. The minimum loss of 0.5 dB/m was measured in the near infrared at 1064 nm. Absorption loss at 2936 nm was 0.88 dB/m with a damage threshold higher than 1.2 kJ/cm2 for 110‐μs‐long pulses making tissue ablation feasible with fibers several meters in length.

Composition dependence of lithium diffusivity in lithium niobate at high temperature

An investigation was performed on the diffusivity of lithium in lithium niobate at 1100 °C in the crystallographic z direction over the composition range from 48.38 to 49.85 mol % Li2O. A vapor transport technique was applied to produce a lithium niobate crystal with a widely varying diffusion‐limited lithium concentration profile. The profile was measured through spatially resolved measurement of the phase‐match temperature for frequency doubling of a Nd:YAG laser. A Boltzmann–Matano analysis was applied to the profile to estimate diffusivity as a function of composition. The validity of the Boltzmann–Matano results was examined through numerical simulation, taking into account finite interfacial mass transport. A dramatic increase of lithium diffusivity with Li/Nb ratio was observed, ranging from roughly 3×10−9 cm2/s at the 48.38 mol % Li2O congruent composition to roughly 50×10−9 cm2/s at 49.85 mol % Li2O.

69% Efficient continuous-wave second-harmonic generation in lithium-rich lithium niobate.

Lithium-rich lithium niobate fabricated by vapor transport equilibration is used to frequency double the output of an injection-locked Nd:YAG laser. Internal doubling efficiencies as high as 69% and powers of as much as 1.6 W are achieved by resonant external cavity second-harmonic generation. No evidence of photorefractive damage is observed at the operating temperature of 234 degrees C.

Growth of Sr0.61Ba0.39Nb2O6 fibers: new results regarding orientation

Abstract Single-crystal Sr 0.61 Ba 0.39 Nb 2 O 6 (SBN:61) fibers of high optical quality have been grown by the laser-heated pedestal growth (LHPG) method. Strontium barium niobate, based on its photorefractive properties, is a promising medium for holographic data storage. The use of a fiber bundle in this application is particularly attractive because each individual fiber can store many (10–30) independent holograms with little cross-talk between adjacent fibers. For this purpose, fibers that are oriented with the c -axis perpendicular to the pulling direction are preffered because they provide a significant increase in photorefractive performance when compared to fibers grown along the c -axis. In this paper we report, to our knowledge, the first stable growth of SBN fibers along the [100] and [110] crystallographic axes. Previous attempts to grow bulk crystals with these orientations using the Czochralski technique have been generally unsuccessful because of diameter instability. We do not encounter this problem with the LHPG method and instead find that [100] and [110] fibers can be readily pulled. Growth conditions, morphology and crystal quality are discussed.

Suppression of mid-infrared light absorption in undoped congruent lithium niobate crystals.

Absorption measurements on 5 mol. % MgO-doped and undoped congruent lithium niobate (LiNbO(3)) crystals revealed absorption bands in the wavelength range of 2500-2800 nm, caused by incorporated hydrogen ions. High-temperature annealing was applied to the congruent LiNbO(3) (CLN) crystals, which decreased the absorption significantly. Then the annealed CLN crystals were periodically poled. As an application of the low-loss annealed CLN crystal, the operation of a 1550 nm pumped singly resonant CW optical parametric oscillator, resonant around 2600 nm, using a periodically poled crystal was demonstrated.