Heterodyne techniques

The capillary wave frequency is detected by an optical heterodyne technique. The laser beam, quasi-elastically scattered by the capillary wave at the liquid-liquid interface, is accompanied by a Doppler shift. The scattered beam is optically mixed with the diffracted beam from the diffraction grating to generate an optical beat in the mixed light. The beat frequency obtained here is the same as the Doppler shift, i.e., the capillary wave frequency. By selecting the order of the mixed diffracted beam, we can change the wavelength of the observed capillary wave according to Eq. (11). 

For very accurate line profile measurements, a heterodyne technique has been developed 240) which can be briefly explained as follows the light, scattered into a cone within the angle 0 b9 (50< 1 °). is focused onto the cathode of a photomultiplier. The photocurrent is proportional to the square of the incoming light amplitude but cannot follow the rapid light frequency. Any beat frequencies, however, resulting from interference between the... 

In Florence, we have chosen an approach that combines laser spectroscopy with the direct frequency measures of the microwave experiments [4]. We take advantage of the obvious consideration that to obtain the FS separations there s no need to precisely know the optical transitions frequencies but just their differences. Thus, if we have two laser frequencies whose difference can be accurately controlled, we may use one as a fixed reference and tune the second across the atomic resonances, as illustrated by Fig. 1. In fact, our approach reverts to an heterodyne technique, where all the transitions are measured with respect to the same reference frequency, that can take any arbitrary but stable value. In the experimental realisation we obtain the two frequencies by phase-locking two diode lasers (master and slave), i.e. phase-locking their beat note to a microwave oscillator [14]. We show in Fig 2 a full-view of the experimental set-up. 

Work is in progress at Oxford to improve this result by wide-band heterodyning techniques which avoid the need for frequency standards. 

Using a cantilever with frequency stimulation with COi (e.g. an a.c. potential at a surface electrode) and co2 (e.g. a stimulation with chopped light) can be performed by choosing a>i—co2=cores. With this or some other heterodyne technique, experiments on the time scale of today s processors can be performed [287, 397-403]. 

Obviously, single-beam measurements cannot characterize collective particle motions because gi2)(q. r) does not depend on v. Dual-beam (heterodyne) techniques (Section III.A.2) measure the real part of g<1)([Pg.231]

According to the literature, narrow band detection in the THz regime often involves heterodyne techniques or Fourier transform spectroscopy, both of which require exotic supplementary hardware. " Our tuned detector involves just a few modifications to the... 

Hydrogen chloride (HC1) has been detected by spectroscopic absorption (Ackerman et al, 1976 Farmer et al, 1976 Raper et al., 1977 Williams et al, 1976 Zander et al., 1996), by absorption radiometry (Eyre and Roscoe, 1977), by in-situ filter sampling and chemical analysis (Lazrus et al, 1975, 1976), by submillimeterwave heterodyne techniques... 

The spectrum of lattice-trapped atoms is recorded using a heterodyne technique. Light fluoresced by the trapped atoms is combined with light (frequency shifted by a modulator) from the laser forming the lattice. When the beams mix on a photodiode they create a beat signal at the difference frequency between the fluorescence and the frequency-shifted laser. The power spectrum of the photocurrent is identical to the fluorescence power spectrum, but centered at radio frequency. This heterodyne technique is not sensitive to the frequency jitter of the laser because the jitter is common between the fluorescence and the laser, which acts as a local oscillator. 

The signals from interstellar space are veiy weak and must be amplified in order to be detected. Direct amplification of the space signals is ineffective, because millimeter-wave electronics generate too much random noise. Instead, the super heterodyne technique is used at millimeter telescopes. In this scheme, the radiation from space, Vsky, is mixed (combined) in a solid state diode, the mixer, with a signal generated at the telescope, the so-called local oscillator , vl.o, or simply LO . The LO is produced by a semiconductor chip, usually a Guim diode. TTie mixer generates an intermediate frequency, vi.f, by... 

In the filter technique or heterodyne technique with spectrum analyzer (see Chapter 4). 

Note that the heterodyne technique introduces no extra terms into the scattered field time-correlation function, as contrasted to the homodyne spectrum of a many-exponential process. 

From Eqs. (4.3.12) it is clear that the homodyne and heterodyne techniques measure... 

Two interesting new techniques have been developed. One, a transient heterodyne technique, due to Dunbar, may be used to determine rate constants for momentum transfer and for charge transfer occurring without momentum transfer (i.e., outside the centrifugal barrier). It is relevant to note here that Dunbar s results, on the two systems which he studied, are consistent with the results of other workers CH4 H- CH4 (Section 3.6.3a) and N2 +... 

With fast electronic counters, frequencies up to a few gigahertz can be measured directly and compared with a calibrated frequency standard, derived from the cesium clock, which is still the primary frequency standard [1316]. For higher frequencies a heterodyne technique is used, where the unknown frequency Vx is mixed with an appropriate multiple tmvr of the reference frequency vr (m = 1,2, 3,...). The integer m is chosen such that the difference frequency Av = - tmvr at the output... 

The next section (7.3.10) presents an over-all discussion of the usefulness of the three-frequency nonlinear heterodyne technique for radar and communications applications. 

The digital results, in particular, may be easily extended in a number of directions. Stochastic signals, rather than sinewave signals, could be treated in the binary communication problem. An extensive treatment of M-ary communications is possible, as is the generalization from a single detector to an array of detectors [7.76-78]. Consideration could be given to the optimum matched filter detector rather than the envelope detector discussed earlier. While the present treatment consists of a per-symbol analysis, prediction could be used to estimate the atmospheric turbulence level over a time period from a particular symbol, for example. In short, the usual variations possible with the conventional heterodyne system may be extended and/or modified for application to the three-frequency nonlinear heterodyne technique. 

Recently, a novel rf-laser double resonance method for optical heterodyne detection of sublevel coherence phenomena was introduced. This so-called Raman heterodyne technique relies on a coherent Raman process being stimulated by a resonant rf field and a laser field (see Fig.l(a)). The method has been applied to impurity ion solids for studying nuclear magnetic resonances at low temperature3 5 and to rf resonances in an atomic vapor /, jn this section we briefly review our results on Raman heterodyne detection of rf-induced resonances in the gas phase. As a specific example, we report studies on Zeeman resonances in a J=1 - J =0 transition in atomic samarium vapor in the presence of foreign gas perturbers. 

The scattered light is imaged onto a detector, where it is superimposed with part of the laser beam. The detector output contains the difference-frequency spectrum Ao) = cjoi — (i> = ( l + s) v, which is electronically monitored with a heterodyne technique. One example is an airborne CO2 laser anemometer that was developed for measuring wind velocities in the stratosphere in order to improve long-term weather forecasts [15.124]. Further examples are measurements of the velocity profiles in the exhaust of turbine engines of planes, in pipelines for gases and liquids, or even in the arteries of the human body. 

In the following we summarize several publications where heterodyne techniques were combined with various photon echo schemes in one or the other way, and were utilized for high-resolution spectroscopy in crystals. These subjects could have also been inserted in Section 7. 

Laser-Microwave Heterodyne Techniques for Spectroscopic Purposes... 

Spectroscopy utilizing tunable laser and microwave sources has been applied widely in exploring atoms, molecules, and condensed matter. Besides the classical areas of optical double resonance and optical pumping the extension of these or related methods to difference frequency measurements in the optical range seems to be of increasing importance. This includes heterodyne techniques. Laser microwave schemes can also play an essential role for the generation of modem frequen( standards. Last but not least, there will be many technical applications like infrared detectors, wavemeters, magnetometers, etc. 

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