a = 2 π Δ k {\displaystyle a={\frac {2\pi }{\Delta k}}}
Δ k = k S P − k x , p h o t o n {\displaystyle \Delta k=k_{SP}-k_{x,photon}}
Δ k {\displaystyle \Delta k}
k S P {\displaystyle k_{SP}}
k x , p h o t o n {\displaystyle k_{x,photon}}
