When the length of cutoff region is so long or the main frequency is so high to approach the cutoff frequency, the main frequency of wave packet is so high to exceed the cutoff frequency.

• 电场频谱图显示波包的频谱明显上移,而且当截止区长度足够大时,或者输入波频率趋近于截止区截止频率时,波包的峰值频率甚至超过了截止区的截止频率。