Joseph E. Schaar

Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs

We generated 1 mW of average output power at 2.8 THz (bandwidth of approximately 300 GHz) in a diffraction-limited beam by placing a 6-mm-long quasi-phase-matched GaAs crystal inside the cavity of a synchronously pumped optical parametric oscillator (OPO). The OPO used type-II-phase-matched periodically poled lithium niobate as a gain medium and was pumped by a mode-locked laser at 1064 nm, with a 7 ps pulse duration, 50 MHz repetition rate, and 10 W average output power. The terahertz radiation was generated by difference frequency mixing between the signal and idler waves of the near-degenerate doubly resonant OPO.

Terahertz Sources Based on Intracavity Parametric Down-Conversion in Quasi-Phase-Matched Gallium Arsenide

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of microstructured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. The OPO used type-II phase-matched periodically poled lithium niobate as a gain medium and was synchronously pumped by a mode-locked laser at 1064 nm (7 ps and 200 nJ at 50 MHz). With center frequencies spanning 0.4-3.5 THz, 250-GHz bandwidth radiation was generated. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21times over the singly resonant oscillator performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW. The GaAs stabilized the DRO by a thermooptic feedback mechanism that created a quasi- continuous-wave train of THz pulses.

Theoretical description and design of a fast-light enhanced helium-neon ring-laser gyroscope

We describe an enhanced rotation sensor involving an active helium-neon (HeNe) ring laser coupled to a passive enhancement resonator, which has been named a fast-light-enhanced HeNe ring-laser gyroscope (RLG). Theoretical rotation sensitivity enhancements as large as two orders of magnitude are presented. The physical effect responsible for the increased rotational sensitivity is the anomalous dispersion of the enhancement resonator, which produces a larger beat frequency as compared to a standard HeNe ring-laser gyroscope (RLG) as the laser cavity is rotated. We present the layout of the fast-light enhanced HeNe RLG, and we provide the theoretical modeling of the enhanced rotational sensitivity. A design is presented for the red HeNe (632.8 nm). The beat frequency is calculated with respect to rotation rate, which defines the useful range of operation for this highly sensitive RLG. Considerations for practical issues including laser-mirror reflectivity precision, unsaturated laser gain, and cavity-length stability are discussed.

Terahertz wave generation in orientation-patterned GaAs using resonantly enhanced schemes

Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient. Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses. Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.

Characterization and control of pulse shapes in a doubly resonant synchronously pumped optical parametric oscillator

The intracavity signal and idler pulses of a low-loss synchronously pumped doubly resonant optical parametric oscillator were characterized experimentally and simulated numerically versus cavity-length detuning. At operation several hundreds of times above threshold, the detunings that maximize the intracavity average power do not necessarily maximize the temporal overlap of the signal and idler pulses, as is desirable for devices making use of intracavity mixing. Independent control of the signal and idler cavity lengths allowed control of the widths and temporal positioning of the pulses. Numerical studies were performed exploring the intracavity power and temporal overlap of the signal and idler pulses under various group-velocity-mismatch conditions. There was good agreement between the experimental and numerical simulation results.

Periodically poled vapor transport equilibrated lithium niobate for visible light generation

The ability to achieve high quality periodic poling in lithium niobate (LN) has allowed quasi-phase-matching to be used for second-order nonlinear optics, leading to experimental demonstration of efficient optical frequency generation throughout its wide transparency range (0.35-4.5 microns). Applications of congruent lithium niobate involving visible or ultraviolet wavelengths are limited to low power or high temperature operation due to the effects of photorefractive damage (PRD) and green-induced infrared absorption (GRIIRA). The standard methods of suppressing PRD include doping with 5 mol-% MgO or ZnO and varying crystal stoichiometry. More recent methods employ a combination of lower doping level and near-stoichiometric composition. We use vapor transport equilibration (VTE) and significantly lower MgO doping (<0.5% in the melt) to obtain near-stoichiometric PRD-resistant crystals with improved parameters for periodic poling compared to the commercially available 5% MgO-doped congruent crystals. An efficient process for periodic poling at room temperature using baked photoresist as a patterned dielectric on one crystal surface with LiCl-solution electrodes was developed for periods as short as 8.3 microns for 0.5% and 7 microns for 0.3% MgO-doped VTE:LN. The quality of periodic poling improves as the MgO concentration is lowered. Stable second harmonic generation of 1.3-W continuous-wave 532-nm radiation was observed near room temperature (43 degrees Celsius, as determined by the phase matching condition) with no sign of degradation in a 1.5-cm long crystal of 0.3-% MgO-doped VTE:LN periodically poled with a period of 7.06 microns.

THz source based on resonantly-enhanced difference frequency generation in periodically-inverted GaAs

We report mW-average-power widely tunable (0.5-3.5 THz) monochromatic THz source based on frequency mixing in periodically-inverted GaAs, between the two closely spaced dasiasignalpsila and dasiaidlerpsila waves, inside the resonant cavity of an optical parametric oscillator.

Tunable THz source based on intracavity parametric down-conversion in quasi-phase-matched GaAs

We developed an efficient frequency-tunable THz source using intracavity mixing between the two resonant waves of a synchronously pumped doubly resonant OPO. Three types of quasi-phase-matched GaAs were utilized: optically contacted, orientation-patterned, and diffusion-bonded GaAs.

Near-degenerate doubly resonant synchronously pumped OPO for intracavity THz generation

We developed a source of terahertz-frequency radiation using intracavity difference-frequency mixing in three types of micro-structured GaAs between the signal and idler waves of a doubly resonant synchronously pumped optical parametric oscillator (DRO). The pump, signal, and idler waves were type-II quasi-phase-matched (QPM) by a 10-mm-long magnesium-doped periodically poled LiNbO3 crystal. The oscillator operated near frequency degeneracy with signal and idler bandwidths of 100-200 GHz. The DRO was pumped by a CW-mode-locked laser with a 1064-nm wavelength, 7-ps pulse width, 50-MHz repetition rate, and average power of 10 W. The cavity round-trip power losses were 4-5% for each wave, and at the largest pump power as much as 120 W of total signal and idler average power was stored inside the cavity. We developed an electronic locking feedback network that provided >15 minutes of stable DRO operation with over 100 W of intracavity average power and power fluctuations of 5-10%. The signal and idler pulse widths were characterized with an extracavity sum-frequency generation process between the pump and resonant OPO pulses. We developed a split-step Fourier propagator to model the temporal properties of the signal and idler pulses, and we found excellent agreement with experimental results.

Tunable Terahertz Generation inside a Synchronously-pumped Optical Parametric Oscillator Using Quasi-Phasematched GaAs

We generated 1 of mW average THz power using quasi-phasematched GaAs as a frequency mixer between an optical parametric oscillator’s (OPO) signal and idler waves. The output frequency was tunable from 0.65-3.4 THz.