Efficient third-harmonic generation in partly periodically poled KTiOPO(4) crystal

A partly periodically poled KTiOPO(4) (KTP) crystal has been designed to integrate quasi-phase-matched second-harmonic generation (QPM SHG) with sum-frequency generation in one crystal for generating a third-harmonic beam. The highest conversion efficiencies of 45% and 3% have been achieved in our experiments for QPM SHG and third-harmonic generation, respectively, by use of picosecond laser pulses at 1.327 microm . We have also discovered that periodically poled KTP has slightly different indices (n(z)) from bulk KTP.     

Efficient resonant frequency doubling of a cw Nd:YAG laser in bulk periodically poled KTiOPO(4)

A cw Nd:YAG laser was quasi-phase-matched frequency doubled by a bulk periodically poled flux-grown KTiOPO(4) crystal in an external resonant cavity. The conversion efficiency and the second-harmonic power with a pump of 225 mW were 55% and 123.5 mW, respectively. The loss of the 1-cm-long crystal, which operated near room temperature, was 1.3% and was dominated by scatter from the cleaved facets of the crystal. No damage, photorefractive or other, was observed at intensites of 370Wmm(-2) , but beam size had to be optimized to eliminate thermal-induced instabilities in the doubling cavity.     

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.     

Quadratic spatial solitons in periodically poled KTiOPO4

The evolution of fundamental beams into quadratic spatial solitons was investigated in a periodically poled bulk KTiOPO4 crystal near and at phase matching. Various soliton properties, such as trapping efficiency as a function of phase mismatch, poling-induced walk-off, and the effects of pulse width, beam width, and input beam quality, were measured. Effects attributed to the presence of cubic nonlinearities were also observed.     

Two-dimensional quasi-phase-matched multiple-cascaded four-wave mixing in periodically poled KTiOPO(4)

A two-dimensional quasi-phase-matched structure is realized in a noncollinear, nondegenerate periodically poled KTiOPO(4) parametric oscillator by utilizing the mutual coherence of two noncollinear pump beams. The mutually coherent pump beams form an interference pattern inside the crystal that is directed perpendicular to the existing quasi-phase-matched grating vector and acts as a parametric gain grating. The cavity itself supports two signal wavelengths around 1550 nm with tunable separation, while the gain grating reduces the operational threshold of the oscillator. Furthermore, with this two-dimensional quasi-phase-matched structure, we can demonstrate the generation of new spectral components through multiple chi((2)):chi((2))-cascaded four-wave mixing processes.     

Temperature-tuned difference-frequency mixing in periodically poled KTiOPO4

4 (PPKTP). We generated 12 μW of radiation tunable around 1.6 μm by difference-frequency mixing of the outputs of a frequency-doubled Nd:YLF laser at 523 nm (240 mW) and a tunable Ti:sapphire laser near 760 nm (340 mW). A temperature tuning rate of 0.73 nm/°C for the generated wavelength and a FWHM temperature acceptance bandwidth of 6.9 °C cm was observed. The effective d₃₃ coefficient was estimated to be ∼5 pm/V. Received: 02 September 1998     

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.     

Femtosecond optical parametric oscillator based on periodically poled KTiOPO(4)

We report a femtosecond optical parametric oscillator based on a periodically poled KTiOPO(4) crystal for which quasi-phase matching is achieved with a 24-microm poling period. The singly resonant parametric oscillator, synchronously pumped by a Ti:sapphire laser at a wavelength of 758 nm, generates a signal at 1200 nm and an idler at 2060 nm. The maximum signal power conversion efficiency of the device is 22% with a pump depletion of 69%. We tune the signal wavelength over a 200-nm band by changing the cavity length. In addition, pump wavelength tuning provides output tunability in the 1000-1235-nm range.     

Continuous-wave optical parametric oscillator based on periodically poled KTiOPO(4)

We report what is to our knowledge the first demonstration of a continuous-wave optical parametric oscillator (OPO) based on periodically poled KTiOPO(4) . The 10-mm-long flux-grown crystal had a quasi-phase-matched period of 9mum . The pump source was a miniature frequency-doubled Nd:YAG laser, and the threshold power of this doubly resonant device was 51 mW. The OPO was operated near room temperature. The signal and the idler wavelengths could be tuned in the range 1037-1093 nm by variation of the crystal temperature (32-38 degrees C) and the cavity length. Unlike in other nonlinear crystals, green-induced infrared absorption was not observed up to the highest pumping intensity of approximately 4.5kW/cm(2) .     

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) .     

Frequency doubling with periodically poled KTiOPO4 at the fundamental wave of cesium D2 transition

We report the continuous wave (CW) second harmonic generation (SHG) with a periodically poled KTiOPO4 (PPKTP) pumped by a diode laser at 852.356 nm, which is exactly resonant on the cesium D2 transition.     

利用周期性极化KTiOPO4晶体参量缩小过程产生明亮振幅压缩光

利用周期性极化KTiOPO4晶体构成的连续准相位匹配简并光学参量缩小谐振腔,获得了注入红外的明亮正交振幅压缩光.参量振荡阈值为35 mW.当抽运光功率为20mW时,测得压缩度为2.23dB,特别是当抽运光功率为8mW时,测得压缩度为2.17dB.     

Observation of triply coincident nonlinearities in periodically poled KTiOPO4

We report the simultaneous quasi-phase-matching of all three possible nonlinearities for propagation along the X axis of periodically poled (PP) KTiOPO4 (KTP) for second-harmonic generation of 745 nm pulsed light from 1490 nm subpicosecond pulses in a PPKTP crystal with a 45.65 microm poling period. This confirms the recent Sellmeier fits for KTP by Kato and Takaoka [Appl. Opt. 41, 5040 (2002)]. Such coincident nonlinearities are of importance for realizing compact sources of multipartite continuous-variable entanglement [Pfister et al., Phys. Rev. A 70, 020302 (2004)], and we propose a new simpler method for entangling four fields on the basis of this triple coincidence.     

基于飞秒激光写制波导的PPKTP晶体倍频实验研究

在PPKTP晶体上进行了飞秒激光刻写波导的实验研究,优化了c-切PPKTP晶体中写制二型波导的工艺参数.当波导长度为10mm,宽度为14.5μm,准相位匹配波长为1064nm时,实现了单模传输.利用该波导对调QNd:YAG激光进行了准相位匹配倍频实验,实现的二次谐波转换效率为39.8%.     

Singly resonant optical parametric oscillator in periodically poled KTiOPO(4) pumped by a Bessel beam

Conical pumping was used in a periodically poled KTiOPO(4) optical parametric oscillator for singly resonant idler generation in a nearly diffraction-limited axial beam. A single signal-idler pair was generated over the whole tuning range by use of asymmetric reflectivity of the OPO mirrors. Pump depletion of 40% and total conversion efficiency of 27% were obtained. Additional OPO tuning capability was demonstrated by adjustment of the angle of the conical pump beam.     

Generation of 740 mW of blue light by intracavity frequency doubling with a first-order quasi-phase-matched KTiOPO(4) crystal

We report on efficient intracavity frequency doubling of a cw diode-pumped Nd:YAG laser on the (4)F(3/2)?(4)I(9/2) laser transition at 946 nm. The nonlinear crystal used in the experiments was a first-order quasi-phase-matched flux-grown KTiOPO(4) crystal (period, 6.09mum ; thickness, 1 mm; length, 9 mm). The fluctuations in the generated second-harmonic wave were lower than 3% at output powers of as much as 500 mW. The overall optical-to-optical efficiency was 5.7%. A maximum output power of 740 mW of blue light was generated, which was stable for only 0.5 min. The decrease the output power at this power level was attributed to heating and thermal lensing in the periodically poled KTiOPO(4) crystal. The short-term behavior of the second-harmonic wave exhibited switching between a cw mode and chaotic intensity fluctuations.     

Simultaneous femtosecond-pulse compression and second-harmonic generation in aperiodically poled KTiOPO(4)

We report both extracavity and intracavity simultaneous second-harmonic generation and compression of pulses at 1.25 mum from a synchronously pumped RbTiOAsO(4) -based optical parametric oscillator, using an aperiodically poled crystal of KTiOPO(4) . The 290-fs input pulses were temporally compressed to 120 fs, with average output powers as great as 120 mW. The experimental results are compared with a numerical model that uses data obtained by characterization of the input pulses by use of the frequency-resolved optical gating technique.     

Watt-level single-frequency tunable Nd:YLF/periodically poled KTiOPO(4) red laser

Intracavity second-harmonic generation of a diode-pumped Nd:YLiF(4) ring laser oscillating on the sigma-polarized (4)F(3/2) - (4)I(13/2) transition (lambda(omega) ~ 1314 nm) with a temperature-tuned periodically poled KTiOPO(4) (ppKTP) crystal is reported, yielding up to 0.92 W tunable (lambda(2omega) = 656-658 nm) single-frequency output. Evidence of self-mode-locking via cascading nonlinearities is observed when the ppKTP is strongly phase mismatched.     

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.