Grating, efficiency

The variation of grating efficiency with wavelength shown in Fig. 2c is based on a simple model of diffraction which is valid for a > A. For finer rulings, polarisafion and resonance effects complicate the situation (Palmer 2000). In the simple case, the efficiency drops to 40% of the peak (blaze) value at the following wavelengths (Schroeder 2000). 

Duhem, O. Henninot, J. F. Warenghem, M. Douay, M., Demonstration of long period grating efficient couplings with an external medium of a refractive index higher than that of silica, Appl. Opt. 1998, 37, 7223 7228... 

Channel-to-channel spectral response correction Normalization factors necessary to correct for channel-to-channel spectral response variations and variations in the spectral transfer effi-of the optical system, e.g., grating efficiency, can be stored in memory and thus provide the means for an automatic spectrum correction, Fig. 4. 

The X3333 component is then responsible for intensity grating efficiency, and X jW, the component is responsible for the polarization grating efficiency. Therefore, the conjugation efficiencies for two different DFWM configurations are... 

In Eq. (3.3) T is transmission, equal to the fraction of light within the Ad and Qd monitored by the spectrometer that reaches the detector. It consists of at least two factors, the transmission of the collection and focusing optics and the transmission within the spectrometer itself. The transmission of the collection optics incorporates any losses from reflection of lenses, mirrors, or a sample cell, while the transmission of the spectrometer incorporates grating efficiency, mirror reflectivity, beamsplitter losses, and the like. In addition, T includes losses from optical filters that may be used between the collection optics and the spectrometer Q [Eq. (3.3)] is the quantum efficiency (c photon ), the fraction of photons reaching the detector producing an electron, which is subsequently counted. 

Figure 8.8. Typical grating efficiency (fraction of incident light diffracted into first order) for different incident polarizations. Each grating design has a different set of curves.

Any changes in reponse from channel-to-channel can be compensated by normalizing each spectrum acquired using a standard spectrum stored in the computer memory. As a result, changes in response of the optical system, such as grating efficiency or detector response can be compensated if known or calibrated. 

The corresponding degenerate cubic susceptibility x (-w w,-w,to) is obtained by comparison of the grating efficiency of measured thin film with that for a standard material, done at the same conditions. As reference we have u-sed CS with known cubic response (-w w,-w,w) = 1.4x10 esu. By defining... 

Figure 10.18 Illustration of a ruled diffraction grating. Top grating orders, with m = 0 indicating the non-dispersive specular reflection. Bottom principle of obtaining increased (blazed) grating efficiency...

Grating Efficiency and Polarization, In connection with grating spectrophotometers it is common practice to specify the blaze angle and wavelength of the gratings employed. It is, therefore, pertinent to discuss the meanings and implications of these terms to avoid the pit-falls of over- or underemphasis of their significance. 

As other factors (e.g. beam splitter versus grating efficiency) were of lesser importance, this meant that overall advantage in signal to noise (S/N) ratio of mid-IR FT instruments over dispersives was basically determined by multiplexing only. Strict comparisons were not possible, though, because FT instruments ran at constant resolution unlike dispersives where resolution varied somewhat across the spectrum. 

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