J. Hellström

Real-time and in situ monitoring of ferroelectric domains during periodic electric field poling of KTiOPO4

We demonstrate a photographic method to view the inversion of ferroelectric domains during the periodic electric field poling of KTiOPO4 by taking advantage of the electro-optic effect and a high speed charge coupled device (CCD) camera. The method is noninvasive and allows monitoring in situ and in real time. The high speed CCD camera opens up the possibility to observe the dynamics of the domains during poling and to probe the samples afterwards to evaluate the grating formed in the material. This monitoring method should be applicable to other ferroelectric crystals, as well.

High-power optical parametric oscillation in large-aperture periodically poled KTiOPO(4).

Electric field poling has been employed to fabricate 3-mm-thick periodically poled KTiOPO>(4) crystal for a high-power optical parametric oscillator. The maximum output power of the singly resonant optical parametric oscillator reached 13 mJ with 45% efficiency when the resonator was pumped with a 2.2-mm-diameter beam from a Q-switched Nd:YAG laser. The influence of the cavity design on the spectral and spatial qualities of the output parametric radiation is also discussed.

First-order type II quasi-phase-matched UV generation in periodically poled KTP

Type II quasi-phase matching was employed for first-order frequency doubling of a single-longitudinal-mode Ti:sapphire laser in periodically poled KTP with a domain-inverted period of 9.01 mum . In a single-pass configuration, 0.44 mW of UV power at 398.8 nm was generated for an input of 780 mW, corresponding to a normalized conversion efficiency of 0.09%W(-1) cm(-1) . The nonlinear coefficient d(24) = 2.82 pm/V was deduced from the measurements. The larger grating period for the type II quasi-phase-matched second-harmonic-generation process than for type I quasi-phase matching, lowers the fabrication constrains in the short-wavelength region and increases the acceptance bandwidth.

Efficient nanosecond optical parametric oscillators based on periodically poled KTP emitting in the 1.8–2.5-µm spectral region

The use of periodically poled KTiOPO(4) as a gain medium in efficient nanosecond optical parametric oscillators pumped by a flash-lamp-pumped Q-switched Nd:YAG laser is demonstrated. Parametric radiation in the 1.8-2.5-mu m spectral region was achieved when the crystal temperature was tuned from 10 to 100 degrees C. A maximum total output energy of 1.8 mJ was obtained at a pump level of 3.5 mJ. Stable operation was achieved, with conversion efficiencies reaching 50%. No gray tracking or laser damage was observed, even for pump intensities of 450MW/cm(2).

Noncollinear optical parametric oscillator with periodically poled KTP

We demonstrate efficient noncollinear low-threshold optical parametric oscillation in periodically poled KTP crystal pumped by the second harmonic of the Q-switched Nd:YAG laser. The noncollinear geometry provides an angular frequency tuning at fixed temperature.

Low-threshold femtosecond optical parametric oscillator based on chirped-pulse frequency conversion

We report what is to our knowledge the first demonstration of a femtosecond optical parametric oscillator based on chirped-pulse frequency conversion in a long crystal of aperiodically poled potassium titanyl phosphate. The minimum pump threshold power was 15 mW, and a signal slope efficiency of 35% was achieved. Continuous tuning from 1190 to 1450 nm was obtained for an average pump power of 800 mW.

Optical parametric amplification in periodically poled KTiOPO(4) seeded by an Er-Yb:glass microchip laser.

An optical parametric amplifier based on periodically poled KTiOPO(4) was used to generate 3-ns pulses at 1.544 mum . The device was pumped by a Q-switched Nd:YAG laser, and the signal output-pulse energy reached 71 muJ ; the maximum gain was 66 dB. The seed source was an Er-Yb:glass microchip laser. A theoretical fit to experimental data gave an effective nonlinear coefficient of 9.7 pm/V , close to the highest values reported for periodically poled KTiOPO(4) . Furthermore, the broad parametric gain observed could be used for broadband pulse amplification.

Efficient femtosecond traveling-wave optical parametric amplification in periodically poled KTiOPO 4

We report, for the first time to our knowledge, high-power femtosecond traveling-wave optical parametric amplification by use of periodically poled KTiOPO(4) . With a single pass through a 4-mm-long sample of 1.23-mm thickness we achieved 40% internal conversion efficiency and 5microJ of single-pulse idler energy near 3.8microm , using only 75microJ of energy from the output of a conventional 1-kHz Ti:sapphire regenerative amplifier. The 210-fs-long idler pulses were almost transform limited. We discuss the specific problems encountered in high-power parametric conversion, such as unwanted quasi-phase-matched upconversion processes for polarization configurations that utilize the largest (d(33)) nonlinear coefficient and the related formation of color centers (gray tracking) in KTiOPO(4) .

Scanning Probe Microscopy Characterization of Ferroelectrics Domains and Domains Walls in KTiOPO4

We use the inverse piezoelectric effect to image artificially made ferroelectric domains of periodically poled KTiOPO4 crystals and KTiOPO4 waveguides using scanning probe microscopy. On applying a high-frequency AC electric field between a gold-coated scanning probe microscope tip in contact with the crystal surface and an electrode below the ferroelectric crystal, the crystal surface oscillates at the frequency of the applied electric field due to the inverse piezoelectric effect. With this technique, and by monitoring the vertical deflection of an AFM tip, which gives contrast between the domains, while the lateral deflection of the tip gives contrast at the domain walls, we determine the apparent domain wall width of the ferroelectric domains to be approximately 20–80 nm for the KTiOPO4. The lateral resolution of this imaging technique is estimated to be about 1 nm.

Low-threshold midinfrared optical parametric oscillation in periodically poled KTiOPO4

In this work we demonstrate low-threshold oscillation in the mid-IR spectral region with periodically poled KTiOPO4 (PPKTP) pumped at 1064 nm by a single-frequency Nd:YAG laser. THe compact pump laser had a diode-pumped passively Q-switched monolithic ring cavity configuration and could generate up to 70 (mu) J, 2 ns pulses. With two PPKTP crystals having ferroelectric domain inversion periods of 37.8 micrometers , the temperature tuning bands for the signal of 1720-1750 nm and 1850-1920 nm have been obtained. The lowest oscillation threshold of 8.3 (mu) J was obtained in a cavity using two mirrors reflecting 99 percent at the signal and the idler wavelengths. The maximum OPO output power of 6 mW and the pump depletion of 39 percent was achieved by driving cavity close to stability limit for the idler field. The output power was substantially increases to 36 mW by using output mirror with 90 percent reflectivity only in the signal band and optimizing pump focusing conditions. The external OPO efficiency in this case reached 21 percent.