Photorefractive Damage Research Articles

High conversion efficiency single-pass second harmonic generation in a zinc-diffused periodically poled lithium niobate waveguide

We report a modified technique for the fabrication of zinc-diffused channel waveguides using z-cut electric-field periodically poled LiNbO3. Unlike previous work, the diffusion was carried out using metallic zinc at atmospheric pressure. By optimizing the thermal diffusion parameters, channel waveguides that preserve the existing periodically poled domain structures, support both TE and TM modes, and enhance photorefractive damage resistance were obtained. Nonlinear characterisation of the channel waveguides was investigated via second harmonic generation of a 1552nm laser with a maximum conversion efficiency of 59%W-1cm-2 at 14.6 masculineC. Using a pulsed source a second harmonic conversion efficiency of 81% was achieved.

Application of Raman spectroscopy for measurement of photorefractive damage profile in LiNbO 3 crystals

We demonstrate that the Raman spectroscopy method may be effectively used for accurate determination of optical damage profiles in LiNbO3-based elements used in the various devices utilizing high-power laser beams. It has been established, that there is an irreversible fraction of the optical damage induced only by the high-intensity (≥25 kW/cm2) illumination for a long time, and it has accumulative character with exposure time in nominally pure crystals. This irreversible effect was related to the optical fatigue of a material. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vapor-transport equilibrated near-stoichiometric lithium tantalate for frequency-conversion applications.

Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by a vapor-transport equilibration process. Because of the resultant increase in photoconductivity and reduction in photogalvanism, the crystals showed no observable photorefractive damage at 514.5 nm up to the highest intensity used, 2 MW/cm2. The crystals also exhibited low green-induced infrared absorption, a Curie temperature of 693 degrees C, and a coercive field of 80 V/mm. The SLT samples were periodically poled with an 8-microm-period grating, permitting first-order quasi-phase-matched second-harmonic generation of 532-nm radiation at 43 degrees C. A 17-mm-long sample generated 1.6 W of continuous-wave output power at 532 nm for 50 h. With 150-ns pulses at a 100-kHz repetition rate in the same sample, 5-W average-power, 532-nm radiation was generated for 1000 h. No damage to the crystal and no aging effects were observed during these experiments.

Tunable THz pulse generation by optical rectification of ultrashort laser pulses with tilted pulse fronts

Optical rectification of ultrashort near-IR laser pulses with tilted pulse fronts and pulse energies of a few μJ in Mg-doped stoichiometric LiNbO3 cooled to low temperature is a powerful technique for efficient generation of THz pulses. The pulse energy critically depends on the Mg doping (necessary for preventing photorefractive damage) and can be easily increased by a factor of three if the MgO content is reduced. Pulse energies up to 400 pJ at repetition rates of 200 kHz and 3.4% quantum conversion efficiency are achieved at 77 K. At 10 K, changing the tilt angle of the pump pulse front results in continuous tuning of the frequency across the 1.0–4.4 THz range. The temporal pulse shapes measured by electro-optic sampling are in good agreement with the signal calculated by a simple theory. This model predicts tunability on a considerably broader range and narrower spectra even at room temperature if GaSe is used instead of LiNbO3. The advantages of the velocity matching technique utilizing tilted pulse fronts are analyzed in comparison with quasi-phase-matching in periodically poled LiNbO3 crystals. The first method provides a ten times higher pulse energy conversion efficiency.

Embryonic Nucleation Method for Fabrication of Uniform Periodically Poled Structures in Potassium Niobate for Wavelength Conversion Devices

A new electric poling concept called the `embryonic nucleation method' for the fabrication of periodically poled structures has been proposed. By applying this method to KNbO3, 1-mm-thick uniform periodically poled KNbO3 (PPKN) with domain inverted period of 35.5 µm and an interaction length of 10 mm has been successfully fabricated by applying only 300 V/mm without the generation of unwanted domains. Furthermore, the reproducibility of the periodic poling process has been greatly improved. SHG using PPKN fabricated by this method was demonstrated and 100 mW second-harmonic laser generation at a wavelength of 532 nm was obtained from 1 W pumping without photorefractive damage at approximately 40°C.

Crystal growth and characteristics of 6.5 mol% MgO-doped LiNbO3

MgO-doped LiNbO3 (MgLN) single crystals of 6.5mol% have been grown by the Czochralski technique. The MgO-doped crystal shows a shift in the absorption edge towards shorter wavelengths and a three orders of magnitude increase in the photorefractive damage resistance compared to the congruent LiNbO3 (CLN) crystal. This material has been periodically poled at room temperature by applying electric field, and a coercive field of about 5kV/mm has been determined, which is about 14 of that for CLN. Periodic poling of 1mm thick 6.5mol% MgLN substrates has been successfully performed by means of an electric field poling process at room temperature.

Wavelength Conversion Using a Quasi-Phase Matched LiNbO3 Waveguide

Wavelength converters using quasi-phase matched LiNbO3 (QPM-LN) waveguides are recognized as key devices for future wavelength division multiplexing (WDM) systems. These converters provide unique characteristics such as the simultaneous conversion of WDM channels, a large signal bandwidth, and transparency as regards modulation format. In this review, we describe waveguide fabrication using the annealed-protonexchange and direct-bonding techniques. A direct-bonded ridge waveguide shows a high resistance to photorefractive damage and a -4.6 dB wavelength conversion using a χ(2)-cascaded pumping scheme is successfully demonstrated. Polarization independent wavelength conversion using a LPE-LN substrate is also described.

A highly damage-resistant zn : linbo/sub 3/ ridge waveguide and its application to a polarization-independent wavelength converter

We have fabricated a periodically poled ridge waveguide using a liquid phase epitaxy (LPE)-grown Zn:LiNbO/sub 3/ substrate. The waveguide can confine both the transverse electric and transverse magnetic mode. Its strong resistance to photorefractive damage enables stable parametric amplification. We demonstrate polarization-independent wavelength conversion using polarization diversity in a single waveguide.

Efficient 340-nm light generation by a ridge-type waveguide in a first-order periodically poled MgO:LiNbO3.

Quasi-phase-matched (QPM) UV second-harmonic generation (SHG) in a periodically poled MgO:LiNbO3 waveguide is presented. A ridge-type waveguide with high nonlinearity and strong resistance to photorefractive damage was achieved by use of an ultraprecision machining technique. By use of this waveguide in 1.4-microm periodically poled MgO:LiNbO3, a first-order QPM SHG device for 340-nm UV radiation was demonstrated. In a single-pass configuration, continuous-wave 22.4-mW UV light was generated for a fundamental power of 81 mW, corresponding to a normalized conversion efficiency of 340%/W.

Direct-bonded QPM-LN ridge waveguide with high damage resistance at room temperature

A direct-bonded 13 mm-long quasi-phase matched LiNbO/sub 3/ (QPM-LN) ridge waveguide is fabricated that exhibits a second-harmonic generation efficiency of 44%/W. The waveguide shows very high resistance to photorefractive damage at room temperature. The difference frequency generation with an efficiency of -17.6 dB in the 1550 nm wavelength band is also demonstrated.

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature.

Amplification of high-bandwidth phase-modulated signals at 793 nm

Amplification of high-bandwidth phase-modulated optical signals from integrated-optics phase modulators at 793 nm is experimentally demonstrated using an injection-locking technique. Off-the-shelf wide-bandwidth integrated-optics modulators are power limited at 793 nm owing to photorefractive damage of the LiNbO3 waveguides. Typical optical input powers for these devices at this wavelength are less than 10 mW with optical output powers typically less than 1 mW. To amplify the outputs of these modulators, we injected the phase-modulated light into an antireflection-coated 100-mW single-mode diode laser. With the injection-locking technique, small-signal gains of 23 dB are demonstrated with good signal fidelity up to bandwidths of 3 GHz. A bandwidth limitation is found at approximately 3 GHz for sinusoidal phase-modulated signals, above which signal fidelity is seriously degraded. This limitation is significantly less than the measured relaxation oscillations of ∼5.6 GHz, suggesting a new limitation to injection locking of phase-modulated signals. Amplification of binary-phase-shift-keyed-modulated signals to 6 Gbit/s is also demonstrated with no bit errors over the 256-bit test sequences.

Blue Light by SHG in Diode Pumped LiNbO3 Waveguides

In this work, second harmonic generation (SHG) in nonlinear channel waveguides by quasi phase matching is reported. The photorefractive damage resistant waveguides were fabricated by Zn-diffusion technique using periodic poled lithium niobate wafers (PPLN). The SH wave, at 397.8 nm, was generated by using a CW diode laser with emission wavelength at 795.6 nm and 1 W peak power was used as fundamental wave. The nonlinear waveguides exhibit a stable conversion even at room temperature with no sign of deterioration induced by the optical damage.

Near-Stoichiometric LiTaO 3 for Bulk Quasi-Phase-Matched Devices

Photorefractive damage at u =532 nm and green-light-induced infrared absorption (GRIIRA) of near-stoichiometric LiTaO 3 (SLT) single crystals were investigated. The SLT crystal even without MgO doping showed high photorefractive damage resistance and suppressed GRIIRA. In addition, a small amount of MgO doped into the SLT crystal almost eliminated the photorefractive damage and GRIIRA. A 3-mm-thick periodically poled SLT (PPSLT) and a 1-mm-thick periodically poled MgO-doped SLT (PPMgSLT) were successfully fabricated and their optical parametric oscillation (OPO) performances were investigated.

Continuous wave waveguide laser at room temperature in Nd3+-doped Zn:LiNbO3

This work reports continuous laser action at room temperature in LiNbO3:Nd3+ channel waveguides, fabricated by Zn diffusion. The absorbed pump power at the threshold was 1.25 mW and a slope efficiency of 20% was obtained. With our available pump power the laser could emit up to 0.14 mW without exhibiting any photorefractive damage.

Parametric wavelength conversion and amplificationusing damage-resistantZn:LiNbO 3 waveguide

Wavelength conversion with a 12 dB parametric gain is demonstrated using a picosecond pulse-to-pulse interaction in a periodically poled Zn:LiNbO3 waveguide. Reduced photorefractive damage enabled intense pumping at a relatively low temperature (~50°C).

Dependence of gray-track threshold of GdYCOB on the crystal growth atmosphere

Gd x Y 1− x Ca 4O(BO 3) 3 (GdYCOB) crystal has considerable potential as a noncritically phase-matched third-harmonic generator (THG) of Nd : YAG lasers. However, photo-induced damages like photo-refractive damage and gray-track occurred during the operation of high-power THG. In this paper, the effect of growth atmosphere on the threshold of photo-induced damage (gray-track) is examined. From our result, the gray-track of GdYCOB, grown in Ar atmosphere has a threshold of 2.3–3.1 MW/cm 2 (355 nm, 6 ns, 10 Hz). The gray-track threshold was increased to 8.4–10 MW/cm 2 when GdYCOB was grown in air ambient.

Low-loss, damage-resistant optical waveguides in Zn-diffused LiNbO3 by a two-step procedure

In the present work the fabrication of high-quality LiNbO3 waveguides, based on Zn diffusion from vapor phase using a two-step diffusion process, has been demonstrated. The optical waveguides fabricated using this method have low propagation losses (less than 0.5 dB/cm measured at 633 nm), as a consequence of the high surface quality obtained. High photorefractive damage resistance is reported, and it has been related to the Zn incorporation into the LiNbO3 crystals. Additional advantages of this waveguide fabrication method due to the low temperature of the process (<900 °C) are to prevent Li out-diffusion and to preserve ferroelectric domain structures.

Efficient frequency conversion of a passively Q-switched Nd:YAG laser at 946 nm in periodically poled KTiOPO_4

Efficient continuous-wave and passively Q-switched diode-laser-pumped Nd:YAG lasers at 946 nm are reported together with results on highly efficient frequency doubling. Periodically poled KTiOPO(4) was employed as the nonlinear material because of its high nonlinearity and its resistance to photorefractive damage. In the blue spectral region we measured 76 mW in continuous wave and 285 mW in the Q-switched mode.

Periodic poling of magnesium-oxide-doped stoichiometric lithium niobate grown by the top-seeded solution method

Magnesium-oxide-doped stoichiometric lithium niobate has been produced using the technique of top-seeded solution growth from a lithium-rich melt. Optical tests, performed with a combination of argon-ion laser lines, have confirmed a previously published result (at 532 nm) that this material has superior resistance to photorefractive damage. This material has been shown, for the first time, to be amenable to periodic poling. Optical parametric oscillator tests have shown that this material maintains the advantages of periodically poled, congruent, un-doped lithium niobate while showing no evidence of photorefractive damage under typical operating conditions. Operating wavelengths as a function of quasi-phase-matching period and temperature have been measured for the optical parametric oscillator, providing useful new information about refractive-index dispersion in this material. This work establishes periodically poled, magnesium-oxide-doped stoichiometric lithium niobate as a viable material for nonlinear optics.