Optical 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). 

We will discuss two active remote-sensing techniques for the atmosphere — the long-path absorption technique and the lidar technique. However, we will first consider a passive technique, in which lasers play an important part in the detection scheme. This optical heterodyne technique is even more frequently used for signal recovery in connection with the active optical remote-sensing methods. The field of laser monitoring of the atmosphere is covered in several monographs and articles [10.70-10.76]. 

The spectral distribution and frequency stability of the output of these stabilized lasers cannot be studied by the methods of conventional high-resolution spectroscopy. Instead it is usually necessary to construct two stabilized lasers, one of which may be regarded as a local oscillator, and to investigate the frequency stability using optical heterodyne techniques. As shown in Fig.13.17 the outputs of both lasers are superimposed coherently on the surface of a square-law detector, such as a photodiode or photomultiplier. The detector output signal then contains a component corresponding to the beat note or frequency difference between the two lasers. The power spectrum of this optical heterodyne signal may be "displayed directly on an r.f. spectrum analyser and its mean frequency determined pre-... 

As in heterodyne detection at r.f. frequencies, the optical heterodyne technique eliminates the problems caused by intrinsic or dark current noise in the detector and the effective signal-to-noise level is limited only by the quantum efficiency of the detector. On the other hand... 

Schematic diagram o apparatus for laser spectroscopy using optical heterodyne techniques.

In condensed-phase CARS, the effects of the nonresonant susceptibility x(3)nr are most profound when a sample with weak Raman modes is embedded in a nonlinear medium. The nonresonant background of the latter can be easily comparable to or larger than the resonant contribution from the sample of interest. This is a situation commonly encountered in biological applications of CARS microscopy. Depending on the experimental situation, the CARS detection sensitivity to weak resonances can then be restricted either by the nonresonant background or by the photon shot-noise [62]. To maximize either the relative or the absolute CARS intensity, nonresonant background suppression schemes [44, 60, 61, 63, 64] and optical heterodyne detection (OHD) techniques [65-67] have been developed during recent years. 

We have outlined how TDFRS not only provides a useful tool for the study of the Ludwig-Soret effect in multicomponent liquids, but can also contribute valuable pieces of information towards solving the puzzles encountered in polymer analysis. Though TDFRS is conceptually simple, real experiments can be rather elaborate because of the relatively low diffraction efficiencies, which require repetitive exposures and a reliable homodyne/heterodyne signal separation. As an optical scattering technique it has much in common with PCS, and the diffusion coefficients obtained in the hydrodynamic limit (q —> 0) for monodisperse solutions are indeed identical. 

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. 

The following section contains a more detailed treatment of the theory behind the nonresonant spectroscopy of liquids. This will be followed by a description of the experimental implementation and data analysis techniques for a common OKE scheme, optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy (22). We will then discuss the application of this technique to the study of the temperature-dependent dynamics of simple liquids composed of symmetric-top molecules. 

The dynamical behavior of molecular liquids is directly observed via femtosecond optical Kerr effect (OKE) measurement in the time domain. The signal intensity, 5oke( ")i obtained by the measurement using an optically heterodyne-detected (OHD) technique (mixing the OKE signal with a local oscillator) can be expressed in the form ... 

A common technique used to enhance the signal-to-noise ratio for weak modes is to inject a local oscillator field polarized parallel to the RIKE field at the detector. This local oscillator field is derived from the probe laser and will add coherently to the RIKE field [96]. The relative phase of the local oscillator and the RIKE field is an important parameter in describing the optical heterodyne detected (OHD)-RIKES spectrum. If the local oscillator at the detector is in phase with the probe wave, the heterodyne intensity is proportional to... 

The major source of noise in single-shot decay is the technical noise introduced by the detection electronics and by fluctuations of the cavity length. Here an optical heterodyne detection technique can greatly enhance the signal-to-noise ratio. The experimental arrangement [40] is illustrated by Fig. 1.20. The output of a cw... 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

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. 

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. 

We also realize that the induced molecular precession requires a deposition of the beam energy in the medium. Since the medium is transparent, this can only happen if part of the beam is downward shifted in frequency. Indeed, the rotation of the polarization ellipse means that the two circularly polarized components of the elliptical polarization have different frequencies (o and (o respectively, with fi) fi) 2ft. Using a heterodyne technique, we were able to measure directly the w component in the output. In this respect, we can also regard the present nonlinear optical effect as a stimulated light-scattering process in which a new frequency component at (o is generated. Details of our experiment and theoretical description will be reported elsewhere. 

New absorption methods, like intracavity spectroscopy, cavity-ring-down and cavity-enhanced spectroscopy, have demonstrated very high sensitivities in laboratory measurements with DLs. An ultrasensitive technique that combines external cavity enhancement and FM spectroscopy has been developed recently. This method, which has been called NICE-OHMS. or noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, is based on frequency modulation of the laser at the cavity free-spectral-range frequency or its multiple. The MDA of 5x 10 1 X 10 cm ) in the detection of narrow... 

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