Receiver heterodyne

Figure Bl.4.2. (A) Basic components of an astronomical heterodyne receiver. The photomicrograph in (B) presents the heart of a quasi-optical SIS mixer and its associated superconducting timing circuits, while the image in (C) shows the fiilly assembled mixer, as it would be incorporated into a low-temperature cryostat (J Zmuidzinas, private conmumication).

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. 

Finally, although heterodyne receiver is basically an amplitude-phase detector, its detectivity as a power receiver is similar to a quantum limited detector ... 

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). 

Fig. 5.2. Schematic r.f. systems, (a) Simple heterodyne circuit, SI determines the pulse length, S2 switches the lens from transmit to receive, and A1 amplifies the reflected signal (b) quasi-monochromatic circuit the two oscillators and the pulse repetition frequency are phase-locked, and the final signal is lock-in detected (courtesy of John...

If vibration is applied both through the cantilever tip (at frequency heterodyne detection. The AFM tip detects the oscillating force at the difference frequency cot — cos, very much like a heterodyne radio receiver. This technique is known as heterodyne force microscopy (HFM Cuberes et al. 2000). Once again, the tip-surface force non-linearity plays a critical role. The low-frequency beating oscillation carries information on the phase of the original high-frequency oscillations. 

In heterodyne detection, this last term of frequency (ojs — cor) is detected (with its phase), while the other terms are filtered out. Heterodyne detection of radio waves uses superheterodyne receivers for light waves, heterodyne detection is really interferometry. 

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... 

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... 

This process of setting a window to limit the range of frequencies admitted to the receiving system is called narrowbanding and can be accomplished in several ways. For fixed frequency operation, it is easy to construct a narrow-band amplifier with a window centered at the desired carrier frequency. This is frequently done, even in commercial spectrometers, and is sufficient if either no other nuclei need to be examined at that field intensity or if no frequency dependent parameters need to be measured. On the other hand, a variable frequency operation can be implemented in several ways. The first is to make (or buy) as many fixed frequency units as needed. This is a simple solution if there is no need for a continuous frequency range and if only a few discrete frequencies are adequate for one s needs. Another is to make the spectrometer tunable, but to keep it a narrow band device. This means that each transmitter and receiver section has to be made tunable, and it is a fairly complicated operation. (The third way is to make it tunable but by a technique known as heterodyning discussed a bit later.)... 

This is an example of a general purpose high power spectrometer for solids. As sketched, it was set up for proton T measurements at 63 MHz but it has been used at various frequencies between 2 and 85 MHz by changing the transmitter and the receiver preamps as well as by using different probes. We do not heterodyne as in the previous example because the receiver is broadbanded. This does mean that the phase shift must be adjusted for each frequency. 

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. 

Fig. 2.11. The generalized optical heterodyne receiver (after Teich [2.122])...

After briefly reviewing conventional optical and infrared heterodyne detection, we examine the behavior of a multiphoton absorption heterodyne receiver. Expressions are obtained for the detector response, signal-to-noise ratio, and minimum detectable power for a number of cases of interest. Receiver performance is found to depend on the higher-order correlation functions of the radiation field and on the local oscillator irradiance. This technique may be useful in regions of the spectrum where high quantum efficiency detectors are not available since performance similar to that of the conventional unity quantum efficiency heterodyne receiver can theoretically be achieved. Practical problems which may make this difficult are discussed. A physical interpretation of the process in terms of the absorption of monochromatic and nonmonochromatic photons is given. The double-quantum case is treated in particular detail the results of a preliminary experiment are presented and... 

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