Noncollinear second-harmonic generation in periodically poled KTiOPO4 excited by the Bessel beam

Frequency doubling in periodically poled KTiOPO(4) excited by the Bessel-like distribution of the fundamental radiation has been investigated. Theoretical analysis and experiments show that the use of noncollinear geometry provides new possibilities for engineering nonlinear interactions in periodically poled crystals. We demonstrate conversion of the ring-shaped fundamental mode to the axial second-harmonic beam with 30% efficiency for 220-muJ Q -switched Nd:YAG pulses.     

Multiple wavelength generation using aperiodically poled lithium niobate

We present a simple method to design aperiodically poled crystals to obtain multiple nonlinear optical processes simultaneously. We use this technique to design and make an aperiodically poled lithium niobate crystal with which we obtained eight distinct sum-frequency generation processes, and another sample with which we obtained four distinct optical parametric generation processes. We also show that the peak effective nonlinearities of aperiodic crystals designed with this technique are larger and their widths are narrower than what can be obtained with a crystal of the same length with separate sections with well-defined poling periodicities.     

Controlling the disorder properties of quadratic nonlinear photonic crystals

We experimentally demonstrate a modulation scheme for disordered nonlinear crystals that combines periodic modulation and disordered sections. The crystal is divided into a set of identical periodically poled building blocks, whereby each block is followed by a short section of random length. We use this scheme to achieve broadband second harmonic generation in KTiOPO4 nonlinear crystals while independently controlling the bandwidth and the center of the converted wavelengths as well as the efficiency of conversion. The trade-off between bandwidth and efficiency is improved in comparison with periodically poled crystals.     

Modeling and Characterization of Border Domain Effects in Periodically Poled Lithium Niobate Crystals Grown by the Off-center Czochralski Technique

We report on the optical characterization of periodically poled lithium niobate crystals grown through the Czochralski off-center technique. The optical diffraction technique pointed out that refractive index changes occur at the border domain. Their effect on the second harmonic generation is therefore investigated by numerical analysis in order to predict the conversion efficiency of the grown crystals.     

Mode conversion in quadratic nonlinear crystals

We propose a novel all-optical, nonlinear mode-conversion scheme based on cascaded three-wave-mixing phase-matched interactions in quadratic nonlinear crystals. We demonstrate the method experimentally by performing all-optical mode conversion of an input 1636 nm Hermite-Gaussian mode from the zeroth order to the first order using two periodically poled LiNbO(3) crystals. Nonlinear mode conversion of an input beam into a higher order, orthogonally polarized output beam can be realized using only one quasiperiodic nonlinear structure. Moreover, it can be enhanced for conversion of complex modes, e.g., Laguerre-Gaussian or Bessel modes.     

Heralding Pure Single Photons Generated with Spontaneous Parametric Down Conversion

We investigate the possibility of generated heralding single photons via spontaneous parametric down conversion using periodically poled lithium niobate crystals with different phase-matching. We use the Schmidt decomposition to calculate the degree of entanglement of photon pairs We simulated the spectral purity, bandwidth, and the joint spectral intensity pictures of degenerate and non-degenerate photon pairs. At last, we obtain an ideal condition of the counter-propagating photons, which can be applied in the quantum communication.     

1.5-microm-band wavelength conversion based on difference-frequency generation in LiNbO3 waveguides with integrated coupling structures

We report wavelength conversion within the 1.5-mum telecommunications band based on difference-frequency generation in periodically poled lithium niobate waveguides with integrated coupling structures. A conversion efficiency of -7 dB and a normalized efficiency of 260%/W are demonstrated. Static tests show that the conversion bandwidth is 72 nm and the conversion efficiency is constant over the 20-dB range of input powers tested.     

Phase-matched nonlinear diffraction

We report on a new (to our knowledge) configuration incorporating both birefringence and quasi-phase-matching, enabling efficient phase-matched nonlinear diffraction in one-dimensional periodically poled nonlinear crystals. We demonstrate the method experimentally, showing an efficient nonlinear diffraction to the first few orders in two types of crystals, MgO doped congruent lithium niobate and congruent lithium niobate, and characterize its efficiency dependence on the fundamental power, the propagation angle, and the crystal temperature. This configuration can increase efficiencies observed in nonlinear diffraction experiments, enables ferroelectric domain characterization by nonlinear microscopy, and can be used to determine the duty cycles of periodically poled nonlinear crystals.     

Broadly tunable second-harmonic generation in periodically poled silica fibers

Broadband tunability of the second-harmonic wavelength is achieved in periodically poled silica fibers. A wavelength tuning range of almost 45 nm of the fundamental wave is demonstrated through mechanical compression tuning of the quasi-phase-matched periodic structure. The uniform strain applied along the entire periodic structure enables the spectral profile and the conversion efficiency of the generated second harmonic to be preserved for the full tuning range. To our knowledge, the achieved tuning range realized through this technique is far greater than that possible with uniform periodically poled crystals.     

Consecutive Parametric Interactions of Light Waves with Nonmultiple Frequencies in Crystals with Irregular Poled Structure

A stochastic theory of quasi-phase-matched consecutive processes, which takes into account peculiarities of the method of creating a nonlinear lattice in crystals, is developed. The influence of aperiodicity of a certain type on the efficiency of the process of consecutive parametric interaction of light waves with nonmultiple frequencies in crystals with irregular poled structure is studied. It has been established that in aperiodically poled nonlinear crystals the regime of exponentially changing intensity of interacting waves can be implemented. A particular case of nondegenerate parametric amplification is also discussed.     

Proposal for all-optical format conversion based on a periodically poled lithium niobate loop mirror

We propose and simulate two novel schemes of 40 Gbit/s format conversion from non-return-to-zero to return-to-zero based on cascaded second-harmonic generation and difference-frequency generation in a periodically poled lithium niobate loop mirror. We investigate the conversion performance, including eye diagrams, conversion efficiency, pulse width, Q-factor, extinction ratio, and tunability. The proposed schemes can perform single-to-dual-channel format conversion. In addition, it is found that simultaneous multichannel format conversion as well as tunable multicasting format conversion can potentially be implemented with great flexibility.     

Holographic recording with reduced intermodulation noise in periodically poled lithium niobate

We show that the use of periodically poled lithium niobate doped with Fe and Y ensures a considerable improvement in the quality of reconstructed images compared with the use of single-domain crystals. This improvement is due to inhibition of intermodulation noise and elimination of optical damage.     

Interaction of light waves in active nonlinear and periodically poled nonlinear crystals

The results of classical and quantum studies of the laser-radiation self-frequency conversion processes in periodically poled active nonlinear crystals are overviewed. The theoretical and experimental results of studying quasi-phase-matched self-frequency doubling and summation of the laser and pump frequencies in an active nonlinear periodically poled Nd:Mg:LiNbO₃ crystal are presented. The possibility of producing frequency-and polarization-entangled states and the sub-Poisson field statistics through a consecutive nonlinear optical frequency conversion in periodically poled nonlinear crystals is considered.     

Single-to-Multiple Channel Wavelength Conversions and Tuning of Picosecond Pulses in Quasi-Phase-Matched Waveguides

We report the single-to-multiple channel wavelength conversions of 1.57-ps pulses based on cascaded secondharmonic generation and difference frequency generation in quasi-phase-matched periodically poled lithium hiebate waveguides. For single-to-single channel wavelength conversion, no external cavity laser is required with use of a fibre ring laser. The conversion efficiency is about -21.44 dB. The converted idler wavelength can be tuned from 1526.4nm to 1537.5nm as the lasing pump wavelength is varied from 1566.1 nm to 1555.0nm. By employing several input pumps, tunable single-to-dual and single-to-triple channel wavelength conversions are experimentally demonstrated.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Light Wave Interactions in Active Nonlinear Crystals with Quadratic Nonlinearity

Recent results in the field of three-frequency wave interaction in homogeneous and inhomogeneous (periodically poled) active nonlinear crystals with quadratic nonlinearity are reviewed. The quasi-phase-matched processes of self-frequency doubling, self-frequency halving, and frequency mixing of the pump and laser radiations are studied. The consecutive quasi-phase-matched processes of the third-harmonic generation and parametric amplification with low-frequency pumping in periodically poled active nonlinear crystals are also investigated. The analysis is carried out for the case of a periodically poled Nd:Mg:LiNbO₃ crystal. Perspectives of using periodically poled active nonlinear crystals in laser devices with self-frequency conversion are discussed.     

Effect of idler absorption on efficiency of optical parametric oscillators

Idler wave absorption is important in optical parametric oscillators (OPOs). We show through both numerical simulation and experiment that proper idler wave absorption coefficient could make OPOs perform better in conversion efficiency. If the coefficient becomes larger or smaller than the proper value, the conversion efficiency would be lower. A periodically poled lithium niobate (PPLN) OPO is set to verify the numerical results. Moreover, different pump power is used in the experiment research. It shows that the proper value is changing with the pump power. It shows that idler wave absorption should be considered comprehensively with pump wave power in efficiency OPO designing.     

Efficient parametric conversion from 1.06 to 3.8 μm by an aperiodically poled cascaded lithium niobate

We experimentally demonstrated an efficient optical parametric oscillator (OPO) with high parametric conversion from 1.0645 to 3.8?μm. An aperiodically poled magnesium oxide doped lithium niobate wafer was designed and fabricated as the nonlinear crystal of the OPO. A linearly polarized acousto-optic Q-switched Nd:YVO4 laser was used as the pump source. High pump-to-idler conversion efficiency of 18.5% was achieved with a slope efficiency of up to 21.5%. When compared with a periodically poled channel fabricated on the same wafer, under the condition of output coupler optimized for the periodically poled lithium niobate based OPO, an improvement of slope efficiency by 28.3% from 15.2% to 19.5% and total efficiency by 12.5% from 13.6% to 15.3% under the highest pump power of 11?W was realized for the pump-to-idler conversion.     

Ultraviolet generation by first-order frequency doubling in periodically poled KTiOPO(4)

First-order quasi-phase-matched frequency doubling at 780 nm in a 1-mm-thick periodically poled KTiOPO(4) crystal with an inverted-domain period of 2.95 microm has been demonstrated for the first time to the authors' knowledge. A normalized conversion efficiency of 1.1%W(-1) cm(-1) has been obtained in cw mode. Experiments show that group-velocity walk-off prevents efficient frequency doubling of femtosecond pulses in KTiOPO(4) in the UV spectral range. However, because of the large optical nonlinearity, periodically poled structures can be efficiently used for frequency doubling of cw and pulsed lasers with pulse lengths as short as ~1 ps .     

Modal phase-matching second harmonic generation in Bragg fiber

Modal phase-matching second harmonic generation in uniformly poled Bragg fiber is theoretically proposed. The very low group velocity of the modes in Bragg fiber near the in-band cutoff frequencies leads to high nonlinear conversion efficiency comparable to that of the periodically poled conversional fiber. The subsequence phase-matched bandwidth reduction by slow light can be retrieved in a certain degree through structure parameters optimization.