Homodyne detection, light scattering

In frequency-domain FLIM, the optics and detection system (MCP image intensifier and slow scan CCD camera) are similar to that of time-domain FLIM, except for the light source, which consists of a CW laser and an acousto-optical modulator instead of a pulsed laser. The principle of lifetime measurement is the same as that described in Chapter 6 (Section 6.2.3.1). The phase shift and modulation depth are measured relative to a known fluorescence standard or to scattering of the excitation light. There are two possible modes of detection heterodyne and homodyne detection. 

Re F stands for the real part of the function F. The function fcs(r) models a slowly varying background, which is usually present in all of the measurements. The constant background term B is measured by the autocorrelator using special time bins with extra delay. / () is the intensity of the local oscillator (may represent scattering due to the interface itself) the term 2/S0// 0 indicates the relative amount of particle-scattered light and reference scattered photons and should not exceed 0.1 for heterodyne detection. The quantity / is an instrumental constant, a value around 0.5 indicating a reasonably optimized system for homodyne detection. 

Correlation spectroscopy is based on the correlation between the measured frequency spectrum S(co) of the photodetector output and the frequency spectrum I((jo) of the incident light intensity. This light may be the direct radiation of a laser or the light scattered by moving particles, such as molecules, dust particles, or microbes (homodyne spectroscopy). In many cases the direct laser light and the scattered light are superimposed on the photodetector, and the beat spectrum of the coherent superposition is detected (heterodyne spectroscopy) [12.81,12.82]. 

Heterodyne detection method for the coherent detection of laser signals, which superposes the optical signal with a reference beam from a coherent light source (local oscillator) in contrast to homodyne detection, the local oscillator is operated at a slightly different frequency than the optical signal heterodyne measurements allow for velocity measurements via Doppler effect and are employed, e.g., in electrophoretic light scattering. 

Single-beam (homodyne) experimental arrangements cannot detect collective particle motions. This results directly from the inability of homodyne techniques to detect phase changes in scattered light. In order to detect phase changes and thus to probe collective particle motions, dual-beam arrangements 150] (Figs. 6 and 7) must be used. 

Equations 5.446 and 5.450 are applicable in the so-called homodyne method (or self-beating method), where only scattered light is received by the detector. In some cases, it is also desirable to capture by the detector a part of the incident beam that has not undergone the scattering process. This method is called heterodyne (or method of the local oscillator) and sometimes provides information that is not accessible by the homodyne method. It can be shown that if the intensity of the scattered beam is much lower than that of the detected nonscattered (incident) beam, the detector measures the autocorrelation function of the electrical held of the scattered light, dehned as... 

The subscripts s and u refer to the scattered and unscattered fields respectively. Heterodyne experiments are technically more difficult than homodyne, and require longer detection times, but can give more accurate results as no assumptions are made about the scattered light. Further, heterodyne techniques avoid inaccuracies that may be incurred in homodyne experiments that unwittingly detect partially heterodyned signals. 

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