(17) ultimately leads to diminished values of χ (3). The requirements on a minimum value of Ω c > Δ Doppler constrains the conversion efficiency that can be achieved because the 1/Ω c 2 factor in Eq. Only pulsed fields have so far been up-converted to the VUV with EIT enhancement. Up-conversion to the UV and vacuum UV has been enhanced by EIT in a number of experiments. This puts additional constraints on the laser pulse parameters. Essentially the number of photons in the pulse must exceed the number of atoms in the interaction volume to ensure all atoms are in the appropriate dressed state. Where f ij are the oscillator strengths of the transitions and NL the product of the density and the length. In contrast, whilst it is advantageous for the field driving the two-photon transition to be single mode (in order to achieve optimal temporal and spectral overlap with the EIT hole induced by the dressing laser), this is not essential since this field does not need to drive the coherence responsible for interference. The latter is not normally suitable for a frequency up-conversion scheme if a significant up-conversion factor is required, e.g., to the vacuum ultraviolet (VUV) however recent experiments report significant progress in cw frequency up-conversion using EIT and likewise a number of other possibilities, e.g., laser-cooled atoms and standing-wave fields, have been proposed.Ī transform-limited single-mode laser pulse is essential for the coupling laser field since a multimode field will cause an additional dephasing effect on the coherence, resulting in a deterioration of the quality of the interference. This is trivially achieved even for unfocused pulsed lasers, but does present a serious limit to cw lasers unless a specific Doppler-free configuration is employed. The coupling laser must be intense enough that its Rabi frequency is comparable to or exceeds the inhomogenous widths in the system (i.e., Doppler width), and a laser intensity of above 1 MW cm −2 is required for a typical transition. The two-photon transition need not be strongly driven (i.e., a small two-photon Rabi frequency can be used) but a strong coupling laser is required. In the case considered with weak probe field, excited state populations and coherences remain small. This is because the Doppler profile follows a Gaussian distribution which falls off much faster in the wings of the profile than the Lorentzian profile due to the natural broadening. The interference effects persist in the dressed profiles providing the coupling laser Rabi frequency is comparable to or larger than the inhomogeneous width. This step is of considerable practical importance as in most up-conversion schemes it is not possible to achieve Doppler-free geometries and the use of laser cooled atoms, although in principle possible, limits the atomic density that can be employed. From this operation the effective values of the susceptibilities at a given frequency are obtained, and these quantities can be used to calculate the generated field. The weighting is determined by the Gaussian form of the Maxwellian velocity distribution. The response of the medium, as characterized by the susceptibilities, must therefore include the Doppler effect by performing a weighted sum over possible detunings. Marangos, in Encyclopedia of Modern Optics (Second Edition), 2005 Propagation and Wave-Mixing in a Doppler Broadened Mediumĭoppler shifts arising from the Maxwellian velocity distribution of the atoms in the medium lead to a corresponding distribution in the detunings for the various members of the atomic ensemble. ![]() Results reported were validated by electrocardiography and accelerometer readings. (2011) demonstrated a high-sensitivity pulsed laser vibrometer used to remotely determine the detailed, time-phased mechanical workings of various parts of the human heart. Laser velocimetry can reliably assess facial muscle activity, associated with emotion and stress at low levels – below the threshold for visible facial deformations. It is responsive to laboratory stressors muscle vibratory activity can be sensed from multiple muscles, including facial muscles, and the comparison is favorable with electromyography. ![]() It can provide advanced recordings of myocardial and vascular performance, of respiratory efforts and sounds, and of tremor activity. Laser Doppler vibrometry can provide advanced measurements in multiple physiological systems relevant to laboratory and field assessment of human stress and emotion ( Tabatabai et al., 2013). Molebny, in Encyclopedia of Modern Optics (Second Edition), 2018 Laser Doppler vibrometryĭoppler effect is used also to measure vibrations where they can exist.
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