Coherence detection

Keywords coherent detection, incoherent source, thermal emission, Shottky noise, photon... 

We investigate here whether coherent detection of optical waves issued from celestrial sources can be processed in a way similar to radio astronomy (Fig. 1). 

Figure 1. Radiowave detector and Optical coherent detection.

Maughan S.M., Kee H.H., Newson T.P., Simultaneous distributed fibre temperature and strain sensor using microwave coherent detection of spontaneous Brillouin backscatter, Meas. Sci. Technol. 2001 12 834-842. 

R.J. Keyes and T.M. Quist, Low Level Coherent and Incoherent Detection in the Infrared M.C. Teich, Coherent Detection in the Infrared... 

The use of high-speed modulated excitation (f> kr + knr) combined with coherent detection methods has resulted in the popular techniques of frequency domain fluorometry, also known as phase-modulation fluorometry. These techniques can be used to determine the temporal characteristics of both fluorescence and phosphorescence and will also be addressed later in this chapter. 

Homonuclear polarization transfer and coherence detection which evolve due to the chemical shift of one nucleus in the tl period... 

Bermel W, Bertini 1, Felli 1C et al (2010) Exclusively heteronuclear NMR experiments to obtain structural and dynamic information on proteins. ChemPhysChem 11 689-695 Kostic M, Pochapsky SS, Pochapsky TC (2002) Rapid recycle C , N and C, C heteronuclear and homonuclear multiple quantum coherence detection for resonance... 

Coherent detection of this signal at angular frequency co will extract the first derivative of the spectral profile, at frequency Ico the second derivative, and so on (Figure 3.9). 

Figure 4.2 Digital modulation strategies for phase-coherent detection. A is the synchronising waveform B C and D E are modulation and detection pairs. For full explanation see text...

Optical heterodyne detection has become of practical use for systems in which the signal source is a laser, for example, in optical communication systems [2.123] and laser radar [2.124]. Teich [2.125] and Arams et al. [2.126] have reviewed the theoretical basis and experimental results. Keyes and Quist [2.127] include a discussion of optical heterodyne detection in their review of coherent detection. 

For these reasons, it is rarely practical to observe microwave or millimeterwave (mmwave) absorptions by simply following the variation of sample transmission with frequency. Rather, one seeks to modulate the absorption by switching it on and off at an audio- or radiofrequency, and to detect any resulting transmitted modulation by phase coherent detection. 

In the earlier decades of microwave spectroscopy, and in all commercial spectrometers on the market at that time, the method of choice was that of Stark modulation. A uniform electric field was applied to the entire sample and switched off and on, typically at a 33 kHz rate. The resulting phase coherently detected spectral frequency scan (Figure 1) shows a spectral line in phase with the switching voltage and a number of electric-field shifted (Stark) components in antiphase with it. 

Hgure 1 A phase coherently detected Stark modulation spectrum of the 4= 3<-2 transition in OCS at 36488.8130 MHz. Three Stark components (negative) appear at varying distances from the unshifted spectral line (positive). (Reproduced with permission from Hollas JM (1992) High Resolution Spectroscopy, figure 4.13, p. 103. Chichester Wiley John Wiley Sons Ltd.)... 

Figure 2 Afrequency modulation spectrum of K =2, K= 1, and K=0 components of the J=7 6 transition of propyne, CH3CCH, at 119.635 MHz, phase coherently detected at twice the modulation frequency, and obtained with a synchronously tuned Fabry-Perot cavity cell. (Courtesy of Wilks AT, with permission.)...

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. 

As with the DSB receiver, achieving coherent detection requires a local oscillator in-phase with the transmitter oscillator. This can be done by transmitting a pilot signal, using a phase-locked loop (PLL) in the receiver circuit, or by simply tuning the local oscillator for the best output signal. 

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