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A Qn on photon statistics (second order correlation function)

  1. Jun 25, 2016 #1
    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.
    Last edited: Jun 25, 2016
  2. jcsd
  3. Jun 25, 2016 #2

    A. Neumaier

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    I think you should provide context by giving a reference.
  4. Jun 26, 2016 #3
    Sorry for the lack of clarity in my post.
    I have been referring mostly to online material as well as Rodney Loudon's "Quantum Theory of Light" which I have had trouble understanding mostly due to my own lack of knowledge in quantum optics. I have found this online response particularly helpful.
    And the same for this wiki article
    However, the questions in my initial post are new questions that have surfaced after reading the above articles.

    The second order correlation function G2(τ) is defined here as = <I(t)*I(t+τ)>/<I(t)>2 where I denotes intensity and the angled brackets refer to time averaged quantities.
    (The Siegert relation says that G2(τ) = 1 + |G1(τ)|2 where G1(τ) is the first order correlation function.)
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