Heterodyne detection method

In the experimental section of this chapter we will present spectra obtained using both homodyne and heterodyne detection methods. The total homodyne detected signal is expressed as... 

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. 

For high accuracy comparison, heterodyne detection and synthesis chains are proven methods but have not been demonstrated in the visible yet. Moreover, since they are somewhat cumbersome and their coverage in the optical spectrum somewhat sparse, it is useful to pursue alternative methods. 

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... 

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. 

The accurate determination of the phase change between the excitation and emission waveforms is the central role in the frequency-domain measurements of luminescence. In the case of prompt fluorophores, the method of choice seems to be heterodyne detection - also called cross-correlation - and subsequent lock-in amplification. In the heterodyne detection, the gain of the photomultiplier is modulated by the frequency m Am where m is the frequency of the excitation modulation. As the consequence the PM signal contains a low-frequency... 

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. 

During the past two years, two alternative methods have been introduced which reach comparable sensitivities via modulation and heterodyne detection techniques. It is interesting to compare the principles of these new techniques with those of the older polarization method. 

The second approach is based on FM sideband spectroscopy, a different modulation method, which is well known in microwave spectroscopy, but whose advantages in the optical region have only recently been demonstrated by J. L. Hall et al. (19), and, independently, by G. C. Bjorklund and collaborators (20,21). In this method, the probe beam is sent through an acoustooptic or electrooptic phase modulator which produces two (or more) FM sidebands of such amplitudes and phases that any constructive or destructive interference effects cancel completely. The intensity of the probe beam, before entering the sample, remains therefore exactly constant. If the sample now changes the amplitude or the phase of any of the sidebands or the phase of the carrier, this delicate balance is perturbed, and the light acquires an amplitude modulation, which can be readily observed with a fast photodiode, followed by rf heterodyne detection. 

Before going into the details of various materials and their third-order NLO properties, it would serve well to have an idea of the characterization techniques used for their study. To study the effect of the real part of third-order susceptibility, Z-scan measurements, degenerate four-wave mixing (DFWM), optical heterodyne detection of optical Kerr effect (OHD-OKE), and differential optical Kerr effect (DOKE) detection are employed. For the study of TPA, techniques such as nonlinear transmission (NLT) method, two-photon excited fluorescence (TPEF) method, and Z-scan measurements are used. The observables from the above-mentioned techniques vary depending on the inherent limitations of the technique. The nature of the light source employed like the central wavelength of the laser. 

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