Optical heterodyne detection

Heterodyne detection is an important technique for low-noise signal recovery. Well-known in the radio-frequency region, it also has its counterpart in the optical regime. The principle of optical heterodyne detection is illustrated in Fig. 10.15. The incoming radiation is mixed in the detector with the radiation from a local oscillator, which could be a diode laser or a CO2 laser. Beats are generated in the detector at the difference frequency between the signal frequency and the local oscillator frequency A narrow-band electronic filter, which only transmits a fixed frequency i/jf, the intermediate frequency, selects the beat frequency is chosen 

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A connnon teclmique used to enliance 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 mtensity is proportional to... 

Tokmakoff A, Lang M J, Larson D S and Fleming G R 1997 Intrinsic optical heterodyne detection of a two-dimensional fifth order Raman response Chem. Phys. Lett. 272 48-54... 

Levenson, M. D., and Eesley, G. L. 1979. Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy. Appl. Phys. 19 1-17. Librizzi, R, Viapianni, C., Abbruzzetti, S., and Cordone, L. 2002. Residual water modulates the dynamics of the protein and of the external matrix in trehalose-coated MbCO An infrared and flash-photolysis study. J. Chem. Phys. 116 1193-1200. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

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. 

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. 

Cong P, Deuel HP, Simon JD. Structure and dynamics of molecular liquids investigated by optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Chem Phys Eett 1995 240 72-78. 

Figure 5 Schematic illustration of optical heterodyne detection of transient gratings. A diffraction grating splits the input pulse into a probe and a reference beam. Diffracted light from the probe beam s interaction with the transient grating propagates collinearly with the reference beam, or local oscillator. (Adapted from Ref. 11.)...

Maznev AA, Nelson KA, Rogers JA. Optical heterodyne detection of laser-induced gratings. Opt Lett 1998 23(16) 1319. 

Femtosecond optical heterodyn-detected optical Kerr effect spectroscopy and low-frequency Raman spectroscopy were used to study the molecular dynamics of selenophene <1998JCP10948>. Femtosecond Kerr effect spectroscopy was also used to examine the third-order polarizabilities of furan, thiophene, and selenophene, which was found to increase from furan to thiophene to selenophene <1996CPL(263)215>. 

We have performed optically heterodyne-detected optical Kerr effect measurement for transparent liquids with ultrashort light pulses. In addition, the depolarized low-frequency light scattering measurement has been performed by means of a double monochromator and a high-resolution Sandercock-type tandem Fabry-Perot interferometer. The frequency response functions obtained from the both data have been directly compared. They agree perfectly for a wide frequency range. This result is the first experimental evidence for the equivalence between the time- and frequency-domain measurements. 

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 ... 

OHD-RIKES(Optical Heterodyne Detected Raman Induced Kerr Effect Spectroscopy)... 

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... 

M.D. Levenson, B.A. Paldus, T.G. Spence, C.C. Harb, J.S. Harris, R.N. Zare, Optical heterodyne detection in cavity ring-down spectroscopy. Chem. Phys. Lett. 290, 335 (1998) M.D. Levenson, B.A. Paldus, T.G. Spence, C.C. Harb, J.S. Harris, R.N. Zare, Frequency switched cavity ring down spectroscopy. Opt. Lett. 25, 920 (2000)... 

M.D. Levenson, G.L. Eesley, Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy. Appl. Phys. 19, 1 (1979)... 

The third principal category of effects is that of wave interaction, which arises from the interaction of the electromagnetic field of the incident radiation with the sensing material. The principal wave interaction effects are optical heterodyne detection and optical parametric effects. Among the others are effects at Josephson junctions and metal-metal oxide-metal contacts. These are listed in Table 2.5 and discussed below. 

Whether optical heterodyne detection should be classed as a wave interaction effect or a photon effect is not obvious. Because it depends upon the interaction of the electric field vector of the signal radiation with that from a reference source, it is listed here as a wave interaction effect. 

Optical heterodyne detection has become of practical use for systems in which the signal source is a laser, for example, in optical communication systems [2.123] and laser radar [2.124]. Teich [2.125] and Arams et al. [2.126] have reviewed the theoretical basis and experimental results. Keyes and Quist [2.127] include a discussion of optical heterodyne detection in their review of coherent detection. 

In addition to optical heterodyne detection and parametric effects, other wave interaction effects include those occurring at Josephson junctions and metal-metal oxide-metal contacts. 

After briefly considering the relevant results pertinent to multiple-quantum direct detection (Sec. 7.2.1), we derive the combination device response for the general multiple-quantum photomixing process (including the important two-quantum case) in Section 7.12. In Section 7.2.3, we obtain the SNR for a receiver using a multiphoton optical heterodyne device, and compare it with the SNR for conventional optical heterodyne detection. The results of a two-photon experiment are presented in Section 7.2.4, while a suggested setup for future experiments, as well as the applicability of the scheme in general, is reserved for Section 7.2.5. 

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. 

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