Simultaneous wavelength tuning and amplitude modulation in QPM optical parametric amplifiers

We repot simultaneous optical parametric amplifiers (OPA) and amplitude modulation of signal pulse in a monolithic periodically poled KTP (PPKTP) by electrically controlling the relative phase between the pump, signal and idler waves in a KTP dispersion section sandwiched between two PPKTP gratings, theoretically. The controlling electric field can be arbitrary direction. For an ultrashort pulse group velocity mismatch, group dispersion and diffraction must be regarded, simultaneously. The solutions of the truncated equations, including the above effects in the two PPKTP gratings of simultaneous frequency tuning and amplitude modulation OPA, were analyzed. It is shown that the intensity of output signal pulse of OPA depends on the thickness of the dispersion section, the magnitude and azimuth of controlling electric field and one evidences periodicity. For the certain angular dispersion there exist the largest frequency bandwidths of optical parametric amplifiers. Certain angular dispersion of a signal pulse can increase the frequency bandwidth, and broadband amplification of ultrashort pulse can be performed effectively.     

Investigation of near-collinear degenerated quasi-phase matching optical parametric amplification using PPKTP crystal

The gain properties of near-collinear degenerated phase-matched optical parametric amplification (OPA) using PPKTP crystal are investigated theoretically. The results indicate that the type-0 phase matching of PPKTP has larger accepted angle and better gain spectrum by tuning crystal temperature or rotating crystal angle.     

基于光束偏转的扫描式宽带光参量啁啾脉冲放大

光参量啁啾脉冲放大(OPCPA)是超短激光脉冲领域的重要技术之一,增大增益带宽对提高OPCPA的转换效率、实现宽带光参量放大具有重要的意义.本文将光束偏转和非共线OPCPA有机结合,提出了基于光束偏转的扫描式宽带OPCPA模型.分析了通过光束偏转来时刻改变非共线角,以保证各频率成分的相位匹配,从而增大增益带宽的基本原理.采用提出的扫描式宽带OPCPA,针对800 nm中心波长、带宽约为100 nm信号光的光参量放大进行了数值计算.结果表明:经过扫描式OPCPA后,信号光的带宽与放大之前几乎相同,光谱没有窄化;扫描式OPCPA比固定非共线角方式的放大极大地增加了增益带宽和转换效率,实现了宽带的光参量放大;要满足信号光各频率成分的相位匹配,达到最大的增益带宽和转换效率,需要尽量减小加载到钽铌酸钾(KTa_(1-x) Nb_xO_3, KTN)电光晶体上的电压抖动和电压延时.     

Double quasi phase matching for both optical parametric oscillator and difference frequency generation

We present simultaneous phase matching of optical parametric oscillator (OPO) and difference frequency generation (DFG), using a noncollinear interaction in periodically poled crystal with single grating. Selecting proper grating period Λ and titled angle ξ between the grating vector G and the optical axis of crystal, the noncollinear scheme provides double phase matching solutions over continuous regions of the midwave infrared spectrum. At certain wavelength regions, for DFG process the group velocities between interaction pulses are matching and the phase matching bandwidth reaches maximum. Selecting the different grating period, the broadband midwave tuning and better gain spectrum can be obtained at different wavelength range. For the certain period grating the conversion efficiency is higher. Hence, using the double phase matching configuration, the broadband tunable midwave infrared wave can be produced in single grating. But the configurations have a relatively narrow angular bandwidth.     

Mid-infrared ZGP OPO pumped by near-degenerate narrowband type-I PPKTP parametric oscillator

The total pulse energy of the signal and idler in a near-degenerate type-I periodically poled KTiOPO₄ (PPKTP) optical parametrical oscillator (OPO) was spectrally confined within a 2 nm spectral bandwidth at 2.13 μm. This was achieved by using a volume Bragg grating as the output coupler. Both the signal and the idler from the PPKTP OPO were then simultaneously used to pump a mid-infrared ZnGeP₂ (ZGP) OPO. The 2 nm bandwidth was narrower than the ZGP crystal acceptance bandwidth and, thus, made efficient conversion in the second OPO possible. A total slope efficiency of 10% from 1.06 μm to the 3.5–5 μm region was demonstrated, generating 250 μJ in the mid-IR with only 3.6 mJ of 1.06 μm pump energy. This corresponds to a Nd:YAG pump to mid-IR conversion efficiency of 7%. PACS 42.65.Yj; 42.72.Ai; 42.40.Eq     

Broadband noncollinear optical parametric amplification without angularly dispersed idler

We report a new scheme for direct generation of broadband angular-dispersion-free mid-IR idler pulses via noncollinear optical parametric amplification when group-velocity matched wavelengths cannot be found and the traditional noncollinear geometry fails to increase the phase-matching bandwidth. The scheme does not require any post-amplification idler angular dispersion compensation. We derive and interpret the condition for broadband amplification and absence of idler angular dispersion. A broadband angular-dispersion-free 2.15 μm idler pulse is generated as an experimental demonstration. We identify the potential of the scheme to generate a broadband 3.5 μm idler, with a bandwidth supporting a sub-two-cycle pulse.     

Numerical study on pulse contrast enhancement in a short-pulse-pumped optical parametric amplifier

We investigate pulse cleaning behaviors in short-pulse-pumped optical parametric amplifiers (OPA). We theoretically study the contrast enhancement of amplified signal pulse and generated idler pulse, and reveal their dependence on the parametric gain in both the regimes of small signal and saturated amplifications. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrasts of the pump and signal pulses in a low gain OPA and increases with the gain. The effects of group-velocity mismatch and group-velocity dispersion on the contrast enhancement are also investigated. The results presented in this paper are of value for pulse cleaning.     

飞秒BBO光参量放大中闲频光二次谐波的产生

由于相位匹配条件和非线性晶体透光范围的限制，400nm蓝光抽运的飞秒β-BaB₂O₄（BBO）光参量放大（OPA）输出的参量光调谐范围有限，很难得到波长小于460nm的蓝光和近紫外光.实验采用1kHz钛宝石九通啁啾脉冲放大器的倍频蓝光作抽运光，超连续白光 作种子光，在Ⅰ类非共线相位匹配条件下，利用宽带的飞秒BBO OPA，在一定的实验参数下 获得了530—810nm放大的信号光，以及810nm—17μm波段范围的闲频光.与此同时 ，还获得了410—700nm连续可调的闲频光的二次谐波，其与闲频光层叠分布，单脉冲能量 为26μJ，转换效率大于5%.仅利用单块晶体的飞秒BBO OPA就可以获得410—810nm连 续可调的飞秒脉冲输出，从而为更多研究和应用的需要提供了重要的光源.对飞秒光参量放 大中闲频光二次谐波产生的条件也进行了理论分析. 关键词： 二次谐波 闲频光 非共线相位匹配 飞秒光参量放大     

Ultrabroad gain in an optical parametric generator with periodically poled KTiOPO4

An optical parametric gain bandwidth of 115 THz at full-width half maximum is generated from a picosecond Ti:sapphire pumped degenerate optical parametric generator. This ultrabroad bandwidth could be obtained by first identifying the wavelength where the nonlinear optical material has zero group-velocity dispersion (GVD). By pumping at half this wavelength the degenerate signal–idler pairs can accommodate ultrabroad bandwidths. The explanation for this is that the group velocities of the signal and the idlers are approximately matched and the GVD is small. However, in order to thoroughly investigate the degeneracy region around 1700 nm we fabricated several periodically poled KTiOPO₄ (PPKTP) crystals with different periods, and also one periodically poled RbTiOPO₄ (PPRTP). Both collinear and noncollinear configurations were employed for broadband parametric generation in this region. It was found that the optimum pump wavelength is in the region between 800 nm to 850 nm for PPKTP, and we could also conclude that a similar performance was found for PPRTP. This work will allow the design of optical parametric devices for generating few-cycle pulses in the spectral region between 1.1 μm and 3.8 μm. PACS 42.65.Re; 42.65.Ky; 42.65.-k     

Simultaneous collinear and non-collinear parametric generation in 1D single grating periodically poled lithium tantalate

We report a detailed study of multiple wavelengths optical parametric generation in a single grating one-dimensional periodically poled lithium tantalate crystal. Simultaneous collinear and non-collinear generations are observed around the pump collinear direction. Similar spectra are found in continuous spatial positions symmetrical to the collinear direction, with decreasing signal and increasing idler wavelengths, associated with decreasing powers when the far-field angle increases. A phase matching scheme is proposed to describe these interactions. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results. Single-pass gain and high pump power density are very likely at the origin of the simultaneous collinear and non-collinear QPM interactions.