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：
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：
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.
