David S. Hum

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.

Amplitude modulation and apodization of quasi-phase-matched interactions

We propose several techniques to modulate the local amplitude of quasi-phase-matched (QPM) interactions in periodically poled lithium niobate waveguides and demonstrate apodization by using each of these techniques. When the hard edges are removed in the spatial profile of the nonlinear coupling, the sidelobes of the frequency tuning curves are suppressed by 13 dB or more, compared with a uniform grating, consistent with theoretical predictions. The sidelobe-suppressed gratings are useful for frequency conversion devices in optical communication systems to minimize interchannel cross talk, while the amplitude modulation techniques in general have potential uses in applications that require altering the tuning curve shapes.

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.

Room-Temperature Stable Generation of 19 Watts of Single-Frequency 532-nm Radiation in a Periodically Poled Lithium Tantalate Crystal

We report on a system that produces 19 W of diffraction-limited radiation at 532 nm through single-pass frequency doubling of the output of a 1064 nm Yb3+-doped fiber MOPA in a periodically poled near-stoichiometric lithium tantalate (PPSLT) crystal. The output of the system was stable at the 19 W level for over one hour with no signs of photorefraction. The green power is believed to be limited by infrared-induced thermal dephasing in the PPSLT crystal.

Linearly polarized, 3.35 W narrow-linewidth, 1150 nm fiber master oscillator power amplifier for frequency doubling to the yellow

A high-power linearly polarized Yb-doped silica fiber master oscillator power amplifier at 1150 nm is reported. It produced 3.35 W cw and 2.33 W of average power in 1 micros pulses at a 100 kHz repetition rate, both with 8 pm linewidth. This is the first report, to the best of our knowledge, of a high-power Yb-doped fiber amplifier at a wavelength longer than 1135 nm. The pulsed output was frequency doubled in a bulk periodically poled near-stoichiometric LiTaO(3) chip to generate 976 mW of average power at 575 nm with an overall system optical-to-optical efficiency of 9.8% with respect to launched pump power.

Quasi-phase-matched second-harmonic generation of 532 nm radiation in 25°-rotated, x-cut, near-stoichiometric, lithium tantalate fabricated by vapor transport equilibration

Quasi-phase-matched second-harmonic generation of 532 nm radiation in 25 degrees -rotated, x-cut, near-stoichiometric lithium tantalate has been performed. Using a face-normal topology for frequency conversion applications allows scalable surface area to avoid surface and volume damage in high-power interactions. First-order, quasi-phase-matched second-harmonic generation was achieved using near-stoichiometric lithium tantalate fabricated by vapor transport equilibration. These crystals supported 1 J of 1064 nm radiation and generated 21 mJ of 532 nm radiation from a 7 ns, Q-switched Nd:YAG laser within a factor of 4.2 of expectation.

2.6-watt average-power mode-locked ceramic Nd:YAG laser

We report a 1.2 at. % Nd:YAG ceramic pumped with an 808-nm laser diode, placed in a 1.92-m cavity, and passively mode-locked at 1064-nm with a 1% modulation depth SESAM. At a pump power of 11.1 W, this laser produced 2.6 W of average power with a slope efficiency of 27%. The pulse length was 26 ps at a repetition rate of 78 MHz. The ceramic exhibited no peak power degradation during a 20-hour test of doubling efficiency with periodically-poled, near-stoichiometric lithium tantalate.

Generating ultra-broadband biphotons via chirped QPM down-conversion

Nonlinear Optics: Nonlinear optical response has enhanced our ability to produce coherent light throughout the optical spectral region.

Pyroelectric and Ferroelectric Study of Polarisation Reversal in Near-Stoichiometric LiTaO3

Polarization reversal ferroelectric characteristics (charge-, current- capacitance- and pyroresponse- voltage loops) were investigated for vapor transport equilibrated near-stoichiometric LiTaO3. The transition from the unipolar state to asymmetric and symmetric inversely polarized structures (IPS) was realized by applying external voltage. By photopyroelectric subsurface thermal-wave probing of IPS the position of charged domain boundary and its thickness were estimated. Low (60 V/mm) coercive field process was considered as connected with low (4 μC/cm2) polarization reversal in gradient regions of domain boundaries. Application of monolithitic IPS for various types of differential pyroelectric detectors is proposed.

Generation of 10.5-W CW 532-nm radiation by SHG in vapor-transport-equilibrated, periodically-poled, near-stoichiometric lithium tantalate

Near-stoichiometric lithium tantalate crystals were produced by vapor-transport equilibration. Control of processing conditions has enabled generation of 10.5 W of CW 532-nm radiation by SHG from 29 W of 1064-nm radiation.