Heterodyne sources

The main source of noise of such a heterodyne detector is the photon noise that takes place at the splitting of the local oscillator. Quantum physicists see this noise as originating from vacuum fluctuation on the input arm. This gives directly the spectral density of noise at input hv/2. 

A noise power equivalent to one photon generates an interference signal which has an amplitude equals to twice the rms photon noise of the source. But as only the in-phase components of the source generates an interference with the local oscillator, the result is that the spectral Noise Equivalent Power of the heterodyne receiver is hv. 

Since a heterodyne receiver is an amplitude and phase detector, it could nicely be used to correlate optical signals received at various remote sites. The local oscillator can be a single laser distributed by optical fiber to the various sites or local lasers that can be synchronized "a posteriori" by reference to a common source (e.g. a bright star). 

Figure 3. Detection of thermal source through multiple point heterodyne...

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. 

In addition to the intrinsic lack of stationarity, many of the fluctuations in the glass relax so slowly that they appear to be static sources of light scattering on the time scale of the data collection. These static contributions will introduce a heterodyne component into the observed relaxation function. If the fraction of the light which is quasi-static exceeds 90%, then the observed relaxation function can be interpreted as a heterodyne case and an analysis can be carried out. However, it is not clear that this limit is ever reached in practice. Only 60% of the light was slowly relaxing at all in polystyrene. If at least 90% of the slowly relaxing part becomes quasi-static the heterodyne case will still apply to the observed part of the relaxation function. For PMMA and PEMA this is unlikely to be the case at any temperature near Tg. 

Taking the absorbed optical power density as source term S = a /(pcp) 1 in the heat equation (5), an analytical expression for the normalized heterodyne diffraction efficiency can be derived as a cascaded linear response [88, 89] ... 

Standard OCT techniques require a heterodyne detection equipment to detect the small variations of the interference fringe intensity of the light source that has been focused and backscattered from the sample. Consequently, owing to the strong localization of the light source onto the sample, this technique allows... 

While heterodyne detection is typically the most sensitive, it is problematic to extend its use to FPA systems. Each detector element in the FPA requires LO power, on the order of 1 mW for Schottky diode-based mixers. For large FPAs operating in the millimeter-wave or terahertz bands, this level of power is currently impractical. Various components including LNAs and mixers are available as MMICs at up to about 140 GHz and may eventually extend to 220 GHz or possibly higher [49], Schottky diode-based mixers are available at frequencies extending well into the terahertz range, to 2.5 THz and possibly higher, and can be used wherever a suitable LO source can be obtained [53],... 

Precise frequencies relative to a frequency standard (at frequency oiq) can be provided by comb generation. If we amplitude or frequency modulate (at frequency 2) a source at the standard frequency, then spectral components at frequencies ioq n 2 (n an integer) are generated. If n and 2 are chosen so that ioq + n 2 a w(unknown) then w(unknown) can be precisely determined by heterodyne methods. The challenge is to make (n 2) very large. 

Detection of radiation at radio telescopes is done using superconducting mixers, which are kept at 4.2 K with a closed-cycle helium refrigerator. Radiation from the sky, Vsky, is combined inside the mixer with a source of radiation at the telescope called the local oscillator (or Vlo)- This technique, called heterodyne mixing is needed because signals from space cannot be... 

Concerning experimental problems, we may conclude that there is no good reason to believe that an unexpected mixing occurs between reflected light or unwanted stray light and the scattered light.So the experiments, carried out in the K region described here, are really free of homodyne-heterodyne problems. Another possible source for the discrepancy between theory and experiment may be the use of the aqueous core thickness /ij in the interpretation of the equilibrium experiments. Whether this is permitted is not clear at the moment. 

At frequencies up to -150-200 GHz, solid-state sources such as YIG-tuned oscillators or Guim diode oscillators are now available with power outputs of up to 100 mW. The harmonic generation of such millimetre-wave sources is relatively efficient for doubling and tripling (>10-15%), but for higher harmonics the power drops rapidly ( (1 THz)< 0.1-10 pW). Nevertheless, harmonic generation was used as early as the 1950s to record the submillimetre wave spectra of stable molecules [33]- Harmonics from optimized solid-state millimetre-wave sources are now used to drive astronomical heterodyne receivers up to 900-1100 GHz... 

The instrument is built on the heterodyne principle with 10 MHz as the intermediate frequency. Both the lock frequency and the spectrometer frequency are derived from the synthesizer, which ensures that any source frequency drift would appear equally in both the lock and the observe channels thus resulting in maintaining the resonance condition. 

In many of the sophisticated experimental techniques applied to photophysical problems, the rapid development of the laser has enabled results to be obtained which were unheard of only a few years ago. These developments have been described adequately in previous and current volumes, but there are two applied uses of the laser which have not yet received significant attention, and to which readers attention is drawn by this short extra section this year. The first is concerned with the remote sensing of atmospheric pollutants. The methods available to achieve this object can be classified as passive, for example the heterodyne detection of thermal emission,209 or active, involving some radiation source. The means of attenuating the intensity of such a source are listed below. 

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