28 September 2011 Re-interpreting "coherence" in light of Non-Interaction of Waves, or the NIW-Principle
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The autocorrelation, or the Wiener-Khintchine, theorem plays a pivotal role in optical coherence theory. Its proof derives from the time-frequency Fourier theorem. The derivation requires either dropping the cross-products (interference terms) between the different filed amplitudes corresponding to different frequencies, or taking time integration over the entire duration of the signal. The physical interpretation of these mathematical steps implies either (i) non-interference (noninteraction) between different frequencies, or (ii) the registered data is valid for interpretation when the detector is set for long time integration. We have already proposed the generic principle of Non-Interaction Waves (NIW), or absence of interference between light beams. The hypothesis of non-interaction between different frequencies was used by Michelson to frame the theory behind his Fourier Transform Spectroscopy, which is correct only when the detector possesses a long integrating time constant like a human eye, a photographic plate, or a photo detector circuit with a long LCR time constant. A fast detector gives heterodyne signal. So, the correlation factor derived by the prevailing coherence theory, and measured through fringe visibility, is essentially the quantum property of the detecting molecules compounded by the rest of the follow-on instrumentation. Low visibility fringes (low correlation factor) does not reflect intrinsic property of light alone; it is a light-matter joint response characteristics. So, we re-define coherence by directly referring to the key characteristics of light beams being analyzed as: (i) spectral correlation (presence of multi frequency), (ii) temporal correlation (time varying amplitude of light), (iii) spatial correlation (independent multi-point source), and (iv) complex correlation (mixture of previous characteristics).
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Chandrasekhar Roychoudhuri, Chandrasekhar Roychoudhuri, } "Re-interpreting "coherence" in light of Non-Interaction of Waves, or the NIW-Principle", Proc. SPIE 8121, The Nature of Light: What are Photons? IV, 81211A (28 September 2011); doi: 10.1117/12.895183; https://doi.org/10.1117/12.895183

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