I am trying to better understand the concept of second order coherence G2(τ) (in particular G2(0)) and a few questions have arisen. Note that I am trying to get a physical idea of what is happening so I would appreciate it if your responses can keep the math to the minimum possible. :) How do we physically think of the Siegert relation / chaotic field limit, i.e. G2(0) = 2 when it seems like the value of G2(0) can actually lie between 1 and infinity? Or a perhaps a better way of phrasing this is - what does it physically mean to say that G2(0) > 2? In relation to qn 1, how should one attempt to classify a laser in the laboratory? Since a laser in practice can never be truly monochromatic, does that automatically imply that 1<G2(0)<=2 for a practical laser? Does the upper bound (chaotic field limit) apply for a practical laser? Is it correct to include this upper bound and model a practical laser as a quasi-chaotic light source with a corresponding (longer than most thermal light sources) coherence time? Most of these ideas seem to be centered upon the condition of continuous intensity, i.e. the analogue of a CW laser, or at least to me, they are better understood when considered in such a manner. My final and real question is: how does the physical meaning and definition of G2(0) and G2(τ) change when we think of a laser pulse? Obviously the idea of a laser pulse implicitly means that such light is no longer monochromatic so it follows that G2(0) cannot be = 1? Beyond that, i have trouble moving further. Please help me out.