GW/cm<sup>2</sup>量级飞秒脉冲倍频的宽谱谐波产生

使用含镁摩尔分数为5%的掺镁铌酸锂晶体对57.4 fs脉冲在1550 nm通信波段进行了Ⅰ型(o+o-e)和0型(e+e-e)的倍频对比实验。对于Ⅰ型倍频, 在4.3 GW/cm2的峰值功率密度下得到了谱宽为28 nm、脉宽为79 fs的谐波脉冲, 转换效率最高达54%; 对于0型倍频, 在3.7 GW/cm2的峰值功率密度下得到了谱宽为2.1 nm的谐波脉冲, 转换效率最高为40%。分别从飞秒脉冲多波长成分的相位匹配(频域)和基波与谐波脉冲的群速度匹配(时域)两个角度, 对倍频过程中基波脉冲和谐波脉冲的演变进行了详细分析。发现同时满足多波长成分相位匹配时, 传播中谐波的谱宽能维持不变; 而仅满足中心波长相位匹配时, 谐波光谱则随着传播长度的增加而逐渐变窄。

Abstract

The experiments of second harmonic generation (SHG) of 57.4 fs pulses at 1550 nm wavelength for type Ⅰ(o+o-e) and type 0 (e+e-e) are achieved using Mg-doped lithium niobate crystal with mole fraction of 5% of magnesium. For SHG of type Ⅰ, the second harmonic pulse with spectrum width of 28 nm and pulse width of 79 fs is obtained at the peak density of 4.3 GW/cm2, and the conversion efficiency is up to 54%. For SHG of type 0, the second harmonic pulse with spectrum width of 2.1 nm is obtained at the peak density of 3.7 GW/cm2, and the conversion efficiency is up to 40%. The key points of the evolution for fundamental pulse and second harmonic pulse in SHG process are analyzed, which are the phase-match of femtosecond pulse multi-wavelength components of fundamental wave in frequency domain and the group-velocity-match of fundamental pulse and second harmonic pulse in time domain. It is found that the spectral width can keep constant in the propagation when the phase-match of multi-wavelength components is satisfied. However, the spectral width will gradually become narrower with the increase of the propagation distance when the phase-match of the center wavelength is satisfied only.

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