A. Alexandrovski

Green-induced infrared absorption in MgO doped LiNbO3

Green-induced infrared absorption (GRIIRA) was investigated by a photothermal technique for undoped and Mg-doped LiNbO3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.

Photothermal common-path interferometry (PCI): new developments

The PCI technique, a modification of photothermal spectroscopy, has become a powerful tool for testing various low absorptive optical materials and components. The current state of the technique and recent progress in extending its capabilities toward the mid-infrared region is presented. A 3.39 μm probe was used for testing and studying various semiconductor materials, such as p-doped GaAs, that can exhibit non-thermal response to the pump beam in addition to the thermal one. A simple theoretical model of the PCI method is shown to describe adequately the experimental data, making it possible to calibrate the setup without using a calibration standard.

Optical properties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium tantalate for frequency conversion

Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by vapor transport equilibration, and several important optical and ferroelectric properties were measured. The effect of vapor transport conditions and surface preparation on reproducible ferroelectric engineering of SLT has been studied. Control of these effects along with dramatic decreases in the sensitivity to photorefractive damage and 532 nm absorption has allowed near-room-temperature generation of 10 W of continuous wave 532 nm radiation by second harmonic generation from 29 W of 1064 nm radiation in a 4 cm long device.

UV and visible absorption in LiTaO3

The effects of reduction, oxidation, Li-enrichment and impurity on LiTaO3 crystals were studied. It is demonstrated that the best LiTaO3 crystals show less absorption than LiNbO3, less photorefraction and no green-light-induced infrared absorption.

Limitations of high-power visible wavelength periodically poled lithium niobate devices due to green-induced infrared absorption and thermal lensing

As nonlinear optics applications of periodically poled lithium niobate (PPLN) scale toward high average powers at visible wavelengths, absorption and thermal effects limit device performance. In second-harmonic generation (SHG) of 532-nm light using 53-mm length PPLN, self-focusing has been observed to limit the green output power to 2 W, with filamentation and sample failure occurring at 2.7 W. In this paper, we examine green-induced infrared absorption (GRIIRA) and thermal lensing in PPLN for 532-nm powers up to 2W, revealing the necessary improvements in this material that are required for high-power visible wavelength applications.

Photothermal absorption measurements in optical materials

Summary form only given. Photo-thermal common-path interferometry, referred to as PCI, has been used to study low absorption phenomena in optical materials with the goal of understanding their behavior under high average power laser illumination. This study included the measurement of low levels of optical absorption in commercially important linear and nonlinear optical materials. The behavior of these materials under high intensity illumination was found to be quite complex, with many showing not only a wide scatter in optical absorption, but also different types of photo-chromic nonlinear behavior that depend on power, time, sample history, etc. Representative data are summarized.

Improved material properties in MgO-doped near-stoichiometric LiNbO/sub 3/ for nonlinear optical applications

Summary form only given.In recent years there has been increasing interest in the use of quasi-phase-matched periodically poled lithium niobate (PPLN) for a variety of frequency conversion applications. We investigate the influence of of MgO doping on the green-induced infrared absorption, photorefraction, and ferroelectric domain nature in near stoichiometric LiNbO/sub 3/, and MgO doped near-stoichiometric LiNbO/sub 3/ are expected to show much improved nonlinear properties since they have considerably reduced photorefraction, and ferroelectric domain switching compared to congruent crystals.

CW gray-track formation in KTP

Summary form only given. Gray track damage in KTP crystals exposed to high power visible radiation is well known. Recently, two parallel absorbing tracks on opposite sides of the pump beam (double-tracks) were found in KTP crystals subject to high average-power high-repetition-rate green laser radiation. We report on the same effect observed with CW-laser radiation. Photothermal common-path interferometry (PCI), a modification of the photothermal technique with interferometric sensitivity, was used to detect double-track formation in the early stage of its development. Two pump beams were used, IR (1064 nm) and green (532 nm or 514 nm). A crossed-beam configuration with both pump beams collinear and probe beam crossing them inside the sample made it possible to resolve bulk and surface absorption in KTP.

Stoichiometric LiTaO/sub 3/ as new nonlinear material for bulk quasi-phasematching

Summary form only given. In recent years there has been increasing interest in the use of quasi-phase-matched nonlinear crystal for a variety of frequency conversion applications. Periodically poled lithium niobate (PPLN) and lithium tantalate (PPLT) have been demonstrated to give efficient second harmonic generation and optical parametric oscillation in both the cw and the Q-switched regimes. However, the performance of QPM devices using PPLN or PPLT has been limited for operation at high peak powers because of difficulty in forming gratings in samples thicker than 1 mm, and at high average power in the visible without resort to operation at high temperatures because of the onset of output beam distortion at high light intensity. These limitations are strongly related to material parameters such as a high electric field of /spl sim/21 kV/mm for ferroelectric domain reversal, and highly variable photorefractive damage resistance and optical absorption in the visible.

Mid-infrared crystalline mirrors with ultralow optical losses

Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The low defect density single-crystal nature of these semiconductor supermirrors enables the lowest mechanical losses and hence unmatched Brownian noise performance [1], which currently limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlenband at long wavelengths [2], crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm) [3], enabling cavity finesse values up to 360, 000 at 1.55 μm at room temperature and as high as 400, 000 at cryogenic temperatures for the same wavelength. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses (scatter + absorption) of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings, with the first use of mid-infrared crystalline mirrors in a cavity-enhanced spectroscopy setup enabling a detailed investigation of the reaction kinetics of the deuterated hydroxyl molecule, OD, and carbon monoxide, CO [4]. Absorption measurements based on photothermal common-path interferometry (PCI) [5] reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.