激光传输---FDTD

一、利用EPOCH研究激光在真空中的传输过程：

 激光波长1.06um，脉冲宽度 duration = 50 * femto，计算100fs。

  duration_w = duration / (2.0 * sqrt(loge(2.0))

begin:laser

  boundary = x_min

  intensity_w_cm2 = 1.0e15

  lambda = lambda0 * cos(theta)

  phase = 0  # -2.0 * pi * y * tan(theta) / lambda0

  #pol_angle = pi / 2.0

  profile = gauss(y,0*micron,4*micron)

  t_profile = gauss(time, duration, duration_w)

end:laser

t_profile 设定高斯分布，初始设置使得在15um处激光强度达到最大值。也就是说t=15um/c=50fs。计算时间为100fs，也就是激光在真空传输100fs的时间，即激光已经传输到100fs*c=30um处。这个点激光在15um处达到最大。测试如下：

传到到30um处激光在15um处达到最大，符合初始设置！

同理，如果按照激光周期设置，效果一样。比如希望激光在达到15um处的时候达到最大，计算得到此时的时间=14.15*laser_cycle(激光周期)。测试如下：

一样的设置！

二、设置圆偏振光：

begin:laser

  boundary = x_min

  profile = gauss(y, 0, sigma_w0)    

  t_profile = gauss(time, 2*sigma_t , sigma_t)         

  phase = 0

  pol_angle = 0        # p-pol = 0.0    s-pol = pi/2    

end:laser

begin:laser

  boundary = x_min

  profile = gauss(y, 0, sigma_w0)    

  t_profile = gauss(time, 2*sigma_t , sigma_t)         

  phase = pi/2.0

  pol_angle = pi/2.0    # p-pol = 0.0    s-pol = pi/2    

end:laser

三、Beam waist is defined as the 1/e spatial extent of the electric field of the pulse. So if you take an intensity FWHM width, you need to divide it by 2*sqrt(ln(2)) to get the intensity 1/e width. You then need to multiply by another sqrt(2) to get the 1/e2 width of the intensity profile, which is equal to the 1/e electric field width.