Efficient femtosecond optical parametric oscillators based on aperiodically poled nonlinear crystals

We discuss the use of aperiodically poled nonlinear crystals to improve conversion efficiency in a synchronously pumped ultrafast optical parametric oscillator. We show theoretically that with these crystals the conversion efficiency can be considerably higher than that obtained when a periodically poled crystal with the same length is used. Moreover, we show that pump threshold can be simultaneously improved by use of longer crystals.     

Generation and detection of terahertz radiation by means of periodically and aperiodically poled crystals

The nonlinear optical methods for generating and detecting terahertz radiation using periodically and aperiodically poled crystals in the quasi-phase-matching mode are analyzed. Two radically different versions of optical pumping are considered: a quasi-continuous mode and pumping by Fourier-limited femto-second pulses. General relations are obtained, which determine the spectra of generated and detected waves through the optical pumping parameters and the crystal generator and crystal detector nonlinear transfer functions. It is shown that, by choosing the optimal spatial modulation of the crystal nonlinear susceptibility, one can control the shape of the generation and detection spectra. An optical scheme is proposed and the optimal parameters of domain structures for quasi-phase-matched generation and frequency-matched detection of forward and backward terahertz waves under quasi-continuous-wave optical pumping are reported.     

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.     

Spectral shaping of femtosecond pulses in aperiodic quasi-phase-matched gratings

We analyze the use of cascading second harmonic interactions in quadratic nonlinear crystals to mould the spectral characteristics of broadband near-infrared femtosecond pulses. Using a genetic algorithm, we optimize the design of the aperiodically poled ferroelectric crystal capable of generating the desired femtosecond infrared pulsed radiation.     

Nonlinear pulse compression by the second-harmonic generation in quasiphase and group-velocity matched samples

We investigate numerically the second-harmonic generation from near infrared 100 fs pulses in periodically and aperiodically poled lithium niobate crystals, taking into account the group-velocity mismatch, the group-velocity dispersion, and self-action effects. For the first time, we show that a tenfold pulse compression can be obtained at both fundamental and second-harmonic frequencies. The mechanism of compression and the pertinent quality factors are discussed.     

Theory of backward second-harmonic generation of short laser pulses in periodically and aperiodically poled nonlinear crystals

We study numerically and analytically the backward second-harmonic generation in periodically and aperiodically poled nonlinear crystals in the quasiphase-matched regime. In our numerical analysis, we apply the optimum control technique based on the shooting method. Due to the fast and efficient numerical technique, we are able to analyze this frequency-conversion process in detail, taking into account various factors ?C the pump intensity, group-velocity mismatch and dispersion, linear phase mismatch, and ??chirp?? of nonlinear grating. Also we derive several approximate solutions for the backward harmonic efficiency employed in the undepleted pump approximation.     

Pulse compression and gain enhancement in a degenerate optical parametric amplifier based on aperiodically poled crystals

We describe theoretically a method of obtaining pulse compression and simultaneously improving amplification in a degenerate optical parametric amplifier by use of quasi-phase-matched engineered crystals. The scheme combines the possibility of increasing the signal spectral bandwidth that is offered by a degenerate parametric amplifier with the ability of engineered aperiodically poled crystals to compensate for group-velocity mismatch.     

稀土掺杂光学超晶格的自变频激光模型与应用

通过结合准相位匹配理论和空间分布自混频（或自倍频）激光模型提出了稀土掺杂光学超晶格材料自变频激光理论模型，该模型计入了弱聚焦的高斯光束空间分布，泵浦耦合以及非理想周期结构的影响，并应用该理论模型对Nd^3 离子掺杂非理想周期极化铌酸锂（Nd^3 :APLN)晶体同步自变频多波长激光进行了模拟计算，计算结果，尤其是阈值、可见激光总输出功率和同步可见激光谱线相对强度分布等与实验结果符合得很好，从而表明该模型的有效性。     

Low-threshold femtosecond optical parametric oscillator based on chirped-pulse frequency conversion

We report what is to our knowledge the first demonstration of a femtosecond optical parametric oscillator based on chirped-pulse frequency conversion in a long crystal of aperiodically poled potassium titanyl phosphate. The minimum pump threshold power was 15 mW, and a signal slope efficiency of 35% was achieved. Continuous tuning from 1190 to 1450 nm was obtained for an average pump power of 800 mW.     

Designer femtosecond pulse shaping using grating-engineered quasi-phase-matching in lithium niobate

The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically poled lithium niobate crystal in the undepleted pump regime. Second-harmonic pulse shapes, including Gaussian, stepped, square, and multiple pulses have been characterized using cross-correlation frequency-resolved optical gating and have been shown to agree well with theory.     

Measurement of the extraordinary refractive index dispersion in the MIR for Mg:Nd:LiNbO3 crystals by the use of quasi-phase-matching in a random 1D domain structure

We present a method of measurement of the extraordinary refractive index dispersion in MIR for periodically and aperiodically poled nonlinear crystals with unknown or uncertain periods. The method is based on the spontaneous parametric down-conversion and is useful for the crystals with domain structure formed directly in the process of growth by Czochralski technique. As an example we measure the extraordinary refractive index of in-growth poled 2 mol% Mg and 1 mol% Nd doped congruent lithium niobate and present the corresponding Sellmeier equation in the range of 0.4–3.6 μm.     

Polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating

We demonstrate polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating (FROG) measurements with a fiber-pigtailed, aperiodically poled lithium niobate waveguide. By scrambling the polarization much faster than the measurement integration time, we eliminate the impairment that frequency-independent random polarization fluctuations induce in FROG measurements. As a result we are able to retrieve intensity and phase profiles of few hundred femtosecond optical pulses with 50 MHz repetition rates at 5.2 nW coupled average power without control of the input polarization.     

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.     

From femtosecond infrared to picosecond visible pulses: temporal shaping with high-efficiency conversion

We present a single-pass frequency doubler that efficiently converts broadband near-infrared femtosecond pulses into narrow-spectrum visible picosecond pulses. An optimal control technique is introduced for properly designing aperiodically poled nonlinear crystals to generate visible pulses of the desired amplitude, phase profile, and carrier wavelength.     

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.     

Background-free collinear autocorrelation and frequency-resolved optical gating using mode multiplexing and demultiplexing in aperiodically poled lithium niobate waveguides

We use mode multiplexing and demultiplexing in apodized aperiodically poled lithium niobate waveguides to enable characterization of picosecond optical pulses in a collinear but background-free way using autocorrelation and second-harmonic frequency-resolved optical gating.     

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.     

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.     

New approaches for the fabrication of photonic structures of nonlinear optical materials

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO₄ crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO₄ crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO₄ (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented.     

Red, yellow, green and blue – four-color light from a single, aperiodically poled LiTaO3 crystal

We demonstrate simultaneous quasi-continuous wave generation of red, yellow, green and blue coherent radiation based on quadruple quasi-phase matching (QPM) frequency upconversion from a single, aperiodically poled LiTaO₃ (APPLT) crystal with a diode-pumped Nd:YVO₄ dual-wavelength laser oscillating at 1342 and 1064 nm. We designed and prepared an APPLT crystal, which can provide four pertinent reciprocals for simultaneous quadruple QPM processes. Namely, frequency doubling of 1342 nm generates red light at 671 nm, sum-frequency mixing of 1342 nm and 1064 nm produces yellow light at 593 nm, frequency doubling of 1064 nm achieves green light at 532 nm and sum-frequency mixing of 1342 nm and 671 nm obtains blue light at 447 nm. PACS 42.65.Ky; 42.79.Nv; 42.55.Xi; 42.72.Bj