Compensation of group-velocity mismatch in the frequency-doubling modelocker

We propose a method for reducing the deteriorating effect of group-velocity mismatch on the frequency-doubling modelocker time response. A birefringent plate is used to introduce a time delay between the fundamental and the second-harmonic pulses which is opposite in sign to that accumulated in the frequency-doubling crystal. In the new arrangement the modelocker may provide pulse compression at a sub-picosecond time scale     

Two-color pulse compression in aperiodically-poled lithium niobate

We experimentally demonstrate engineerable compression of two-colored pulses in a linearly-chirped quasi-phase-matching grating. Quadratic solitons generated from fundamental input are reshaped through cascaded parametric processes of second-harmonic generation (SHG) and the back-conversion. We use type-I (e: o + o) SHG geometry in a 50-mm-long aperiodically-poled MgO:LiNbO₃ device to satisfy the group-velocity matching condition. Simultaneously compressed fundamental and SH pulses of about 55-fs duration with small pedestal are generated from the fundamental input pulses of 95-fs duration.     

Broadband quasi-phase-matched second-harmonic generation in MgO-doped periodically poled LiNbO_3 at the communications band

A novel scheme for broadband second-harmonic generation was developed with type I quasi phase matching by use of an off-digital nonlinear optical coefficient d(31) . This was achieved by matching of the group velocities of the second harmonic and the fundamental, while the phase velocities were quasi phase matched. The desired group-velocity dispersion could be obtained with LiNbO(3) doped with 5 mol. % of MgO. Efficient second-harmonic generation with a bandwidth of 52 nm was obtained for the fundamental wavelength centered at 1566 nm in MgO-doped periodically poled LiNbO(3) , compared with a bandwidth of 1.3 nm for the conventional scheme using d(33) . Efficient frequency doubling of an ultrafast pulse is expected without group-velocity walk-off or pulse distortion.     

Demonstration of high conversion efficiency to second harmonic in a wide tuning range

This article discusses the second-harmonic generation (SHG) of tightly focused fs laser pulses in a BBO crystal. It was found theoretically and demonstrated experimentally that the spatial walk-off (SWO) and group-velocity dispersion (GVD) effects can suppress the pulse distortion and gain depression of SHG due to group-velocity mismatch (GVMM), thus keeping a reasonable pulse waveform and conversion efficiency simultaneously in a relatively wide tuning range. Using this approach, the SHG efficiency of a 2 mm long BBO crystal in a reflecting focusing geometry was enhanced significantly.     

Low-voltage tunable second-harmonic generation in an x-cut periodically poled lithium niobate waveguide

We report a new method of tuning the second-harmonic signal generated in a periodically poled lithium niobate (PPLN) waveguide. This technique relies on the recent progress in the fabrication of PPLNs on x-cut substrates along with periodical coplanar electrodes to reduce the tuning voltage while preserving the conversion efficiency. Our scheme exploits a type I interaction implemented in a titanium waveguide to impede cancellation of the electro-optical coefficients and to minimize the group-velocity mismatch between the first- and second-harmonic modes. The tuning range covers 58 nm with an applied voltage of +/-150 V.     

Solitonlike pulse shortening in a femtosecond parametric amplifier

We demonstrate numerically that simultaneous second-harmonic generation and moderate group-velocity mismatch can lead to spontaneous nonlinear shortening during parametric amplification. As an experimental proof we present the results of twofold self-compression of signal pulses in a beta-barium borate parametric amplifier pumped by either 40- or 80-fs pulses from a Ti:sapphire laser.     

High-efficiency blue generation by frequency doubling of femtosecond pulses in a thick nonlinear crystal

We have demonstrated highly efficient frequency doubling of femtosecond pulses in a thick, noncritically phase-matched KNbO(3) crystal under conditions of large group-velocity mismatch. At low power we observed a slope efficiency of ~300% nJ (-1) for harmonic conversion, and at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusing dependence for our conditions of large group-velocity mismatch is considerably different from that obtained for frequency doubling of continuous-wave light. We have also demonstrated that one can tune the spectral width of the generated blue light by varying the focusing conditions.     

Frequency doubling of multi-terawatt femtosecond pulses

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam. PACS 42.65.Ky; 42.65.Re     

超短激光脉冲二倍频理论分析

采用快速傅里叶变换（FFT）算法和Rung-Kutta法对KDP晶体中超短激光脉冲二倍频过程作了研究。详细讨论了群速度失配、群速度色散和三阶非线性相位调制效应对倍频光脉冲波形、光谱及转换效率的影响。研究结果表明：在超短激光脉冲二倍频中，三阶非线性相位调制效应可使二倍频光脉冲形状发生畸变，光谱明显展宽且转换效率降低；当入射基频光功率密度大于100GW/cm2时，三阶非线性效应是影响倍频过程的主要因素。     

Simultaneous second-harmonic generation with two orthogonal polarization states in periodically poled KTP

Simultaneous generation of second-harmonic light with orthogonal polarizations in the blue spectral region is demonstrated by use of typeII and typeI quasi-phase matching for the nonlinear coefficients d(33) , d(24) , and d(32) in periodically poled KTP. The second-harmonic power ratio in both polarization states can be tuned by the fundamental polarization and (or) the periodically poled KTP temperature. Femtosecond pulse frequency-doubling efficiencies of as much as 39.5% W (-1) and 8.1% W (-1) are demonstrated in 7-mm periodically poled KTP for typeI and typeII processes, respectively. The efficiency limitations caused by various effects of the group-velocity mismatch are discussed.     

Phase matching limitation of high-efficiency second-harmonic generation in both phase- and group-velocity-matched structures

In this paper, we studied efficient second-harmonic generation (SHG) of femtosecond pulses in both phase- and group-velocity-matched structures. Obtained results show that phase matching becomes more critical under conditions required for high levels of conversion efficiency. And the imperfect phase mismatch caused by mismatched group-velocity dispersion (GVD) will limit conversion efficiency as well as bandwidth of generated second-harmonic (SH) pulses. The spectral characteristics of the generated SH pulses and its conversion efficiency in the strong pump regime are investigated in detail. The acceptance bandwidth of nonlinear crystal in the high-efficiency SHG is redefined in the paper, and the definition is much closer to the practical application of design.     

Elliptic X-shaped light bullets

Elliptic X-shaped light bullets, which are generated as nondiffraction elliptic Bessel-like beams with central humps of elliptical shape during the propagation of asymmetric input ultrashort pulses in normal dispersive quadratic media, are demonstrated to be quite stable against wave-packet breakup due to asymmetric input or anisotropic diffraction. Walking elliptic X-shaped light bullets can be observed without spatial wave-packet breakup for strongly coupled fundamental-wave and second-harmonic ultrashort pulses with nonvanishing group-velocity mismatch.     

Matched second-harmonic generation of ultrashort laser pulses in photonic crystals

It is shown that one-dimensional photonic bandgap structures are capable of simultaneously satisfying the phase and group-velocity matching conditions for second-harmonic generation involving extremely short light pulses. When these conditions are satisfied, an optical frequency doubler utilizing photonic bandgap structures provides a means for increasing the rate of growth of the second-harmonic signal as a function of the nonlinear-optical interaction length relative to structures designed for quasi-matched interactions and affords possibilities for enhancing the frequency doubling efficiencies independently of the matching length in the bulk nonlinear material. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 800–805 (25 December 1999)     

Tuning quadratic nonlinear photonic crystal fibers for zero group-velocity mismatch

We consider an index-guiding silica photonic crystal fiber with a triangular hole pattern and a periodically poled quadratic nonlinearity. By tuning the pitch and the relative hole size, second-harmonic generation with zero group-velocity mismatch is found for any fundamental wavelength above 780 nm. The nonlinear strength is optimized when the fundamental has maximum confinement in the core. The conversion bandwidth allows for femtosecond-pulse conversion, and 4%-180%W(-1)cm(-2) relative efficiencies were found.     

单模光纤中基于交叉相位调制的压缩脉冲对产生

基于波长位于反常色散区强泵浦脉冲的交叉相位调制,通过对波长位于正常色散区的弱脉冲在单模光纤中的传输进行数值模拟,得出了一种压缩脉冲的新方法.计算结果表明,在零色散波长位于弱脉冲波长和泵浦脉冲波长中间附近的情况下,交叉相位调制(XPM)与群速度色散(GVD)的共同作用能使弱脉冲演化成比其初始宽度窄得多的脉冲对.同时我们还发现,泵浦脉冲的相对于信号脉冲初始峰值功率愈高,初始相对宽度越小,所得弱脉冲的压缩比和压缩后脉冲峰值功率愈高,压缩到最窄时所需光纤长度越短.最后对基于这一压缩的物理机制作了详尽的分析.     

Measurement of ultrashort laser pulses using single-crystal films of 4-aminobenzophenone

Single-crystal thin-film of an organic second-order nonlinear optical material, 4-aminobenzophenone (ABP), is used to measure the pulsewidth of a Ti-Sapphire laser producing 45 fs pulses at 1 kHz repetition rate, by the non-collinear second-harmonic generation (SHG) intensity autocorrelation technique. These films are suitable for measurements over a broad wavelength range, down to 780 nm, due to their wide optical transparency. The single-crystal film with thickness (3 μm) less than the coherence length requires no phase-matching for efficient broadband SHG. Pulse walk-off due to group-velocity mismatch (GVM) and temporal broadening of the pulses due to group-velocity dispersion (GVD) are found to be negligible. These effects have been estimated for pulse width down to few-cycle pulses (10 fs), and the analyses show that these films can be used to characterize such ultrashort optical pulses.     

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.     

Limiting efficiencies of second-harmonic generation and cascaded χ processes in quadratically nonlinear photonic nanowires

Quadratically nonlinear waveguides with subwavelength core dimensions are shown to provide limiting efficiencies of second-harmonic generation and three-wave mixing, as well as cascaded χ⁽²⁾ nonlinear-optical interactions. Small-core waveguides made of high-χ⁽²⁾ materials, such as gallium arsenide and indium phosphide, are shown to allow anomalous dispersion to be achieved within the range of wavelengths from 1.3 to 2.0 μm, with the wavelength of zero group-velocity dispersion controlled by the size of the waveguide core.     

Nonlocal explanation of stationary and nonstationary regimes in cascaded soliton pulse compression

We study soliton pulse compression in materials with cascaded quadratic nonlinearities and show that the group-velocity mismatch creates two different temporally nonlocal regimes. They correspond to what is known as the stationary and nonstationary regimes. The theory accurately predicts the transition to the stationary regime, where highly efficient pulse compression is possible.     

Characteristics of a new-type SHG crystala β-BaB2O4 in the femtosecond region

The optical characteristics of a new nonlinear crystal β-BaB₂O₄, including the group-velocity dispersion and the nonlinear coefficient in the femtosecond region, are investigated through the second-harmonic generation (SGH). The performances of β-BaB₂O₄ relative to KDP are evaluated from both calculations and measurements. From the measurements using the crystals with different thicknesses, the influence of the group-velocity mismatch of β-BaB₂O₄ on the broadening of the correlation width is revealed, and is found to be smaller than that of KDP. Overall performance of the SHG conversion efficiency of β-BaB₂O₄ including all affecting factors is evaluated to show higher values by a factor of 10 to 10² than that of KDP depending on operating conditions.