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利用由预估校正(PC)-时域有限差分(FDTD)法求得的不含慢变包络近似(SVEA)和旋转波近似(RWA)的全波Maxwell-Bloch方程的数值解,研究了飞秒啁啾Gauss型激光脉冲(以下简称啁啾脉冲)在稠密Λ型三能级原子介质中的传播.研究表明,啁啾系数(C)的正负及大小的变化对脉冲传播特性有显著的影响,而且这个影响与脉冲面积的大小密切相关.面积小于4π的啁啾脉冲,在介质中传播时不发生分裂,且啁啾脉冲逐渐演化为一个近似的无啁啾(C=0)脉冲,这一特点不随啁啾系数的改变而改变,但啁啾系数的变化将改变脉冲的振幅和群速度.正啁啾(C>0)情况,振幅和群速度随C的增大而减小;负啁啾(C<0)情况,振幅和群速度则不随C的绝对值的增大而单调地增加或减小.面积等于和大于4π的啁啾脉冲,在介质中传播时都将分裂成不同数目和形状的子脉冲,脉冲分裂的早晚和数目决定于啁啾系数的正负及大小.但是,两种情况下脉冲分裂方式大不相同,啁啾系数影响也不同.脉冲面积等于4π时,大啁啾系数导致子脉冲数目增加,而脉冲面积大于4π时,大啁啾系数导致子脉冲数目减少.不管脉冲面积大小,啁啾系数正负及大小的变化都对介质粒子数布居产生明显的影响。
Using the numerical solution of the full-wave Maxwell-Bloch equation without the slow-change envelope approximation (SVEA) and the spin-wave approximation (RWA) obtained from the PC-FDTD method, Femtosecond chirp propagation of Gauss-type laser pulses (hereinafter referred to as chirped pulses) in a dense Λ-type three-level atomic medium. Studies have shown that the chirp coefficient (C) changes in positive and negative and pulse propagation characteristics significantly , And this effect is closely related to the size of the pulse area.A chirped pulse with an area of less than 4π does not split when propagating in the medium and the chirped pulse gradually evolves into an approximate chirp-free (C = 0) pulse , But this characteristic does not change with the change of chirp coefficient, but the change of chirp coefficient will change the amplitude and group velocity of pulse.The positive chirp (C> 0), amplitude and group velocity decrease with the increase of C ; Negative chirp (C <0), the amplitudes and group velocities do not monotonically increase or decrease with increasing absolute value of C. Chirped pulses with areas equal to and greater than 4π will split when propagated in the medium Into a different number and shape of the sub-pulse, pulse split sooner or later and the number depends on the chirp coefficient of positive and negative and However, in both cases, the mode of pulsation is very different and the effect of chirp coefficient is different.When the pulse area is equal to 4π, the large chirping coefficient leads to the increase of the number of sub-pulses, while the large chirping coefficient leads to The number of sub-pulses is reduced.No matter the pulse area size, the positive and negative of the chirp coefficient and the change of the size have a significant effect on the population of the medium particles.