Total finesse

Mechanical force and surface chemical finesse must be used to overcome van der Waals forces, which may account for as much as 75% of the total adhesion ( 3, 4) (Figure 16). A rapidly rotating fiber brush is often used to produce a local cloud of toner, which is then drawn away through a vacuum filter. The bristle tips may be "flicked" against a bar or edge to free them of dust. Teflon fiber brushes ( 5), stearate wax dispensers ( 6), and fluorocarbon powder ( 7.) have been effectively used to deposit thin low-energy films on the photoconductor surface to facilitate release. 

The total enhancement integrated over wavelength, rather than the enhancement at the resonance wavelength, is relevant for many practical devices. On resonance, the emission is enhanced along the axis of the cavity. However, sufficiently far off resonance, the emission is suppressed. Because the natural emission spectrum of the active medium (without a cavity) can be much broader than the cavity resonance, it is, a priori, not clear whether the integrated emission is enhanced. To calculate the wavelength-integrated enhancement, the spectral width of the cavity resonance and the spectral width of the natural emission spectrum must be determined. The resonance spectral width can be calculated from the finesse of the cavity or the cavity quality factor. 

Total finesses of the order of 50 can be obtained routinely for A./200 plates with R = 98%. Higher reflectivities only lower the transmission without appreciably increasing the finesse because other factors become dominant. 

Ethyl acetates of fatty acids have very pleasant odors of wax and honey which contribute to the aromatic finesse of white wines. They are present at total concentrations of a few mg/1. 

Since we have assumed an ideal plane-parallel plate with a perfect surface quality, the finesse (4.53a) is determined only by the reflectivity R of the surfaces. In practice, however, deviations of the surfaces from an ideal plane and slight inclinations of the two surfaces cause imperfect superposition of the interfering waves. This results in a decrease and a broadening of the transmission maxima, which decreases the total finesse. If, for instance, a reflecting surface deviates by the amount X/q from an ideal plane, the finesse cannot be larger than q. One can define the total finesse F of an interferometer by... 

This illustrates that high-quality optical surfaces are necessary to obtain a high total finesse [4.32]. It makes no sense to increase the reflectivity without a corresponding increase of the surface finesse. In our example the imperfect parallelism was the main cause for the low finesse. Decreasing the wedge angle to 0.1" increases the wedge finesse to 40 and the total finesse to 27.7. 

The total finesse of the confocal FPI is, in general, higher than that of a plane FPI for the following reasons ... 

The total finesse of a confocal FPI is therefore mainly determined by the reflectivity R of the mirrors. For R = 0.99, a finesse F = n R/( — R) 300 can be achieved, which is much higher than that obtainable with a plane FPI, where other factors decrease F. With the mirror separation r = d = 3 cm, the free spectral range is 3 = 2.5 GHz and the spectral resolution is Au = 7.5 MHz at the finesse F = 300. This is sufficient to measure the natural linewidth of many optical transitions. With modem high-reflection coatings, values of F = 0.9995 can be obtained and confocal FPI with a finesse F > 10" have been realized [4.41]. 

In Sect. 4.2.10 we saw that for a given resolving power the spherical FPI has a larger etendue for mirror separations r > /Ad. For Example 4.19 with D — 5 cm, d = 1 cm, the confocal FPI therefore gives the largest product RU of all interferometers for r > 6 cm. Because of the higher total finesse, however, the confocal FPI may be superior to all other instruments even for smaller mirror separations. 

Generally speaking, other losses such as diffraction, absorption, and scattering losses decrease the total finesse. Realistic values are F = 50—100, giving for Example 5.6 a resonance halfwidth of the passive resonator of about 2 MHz. 

A HeNe laser with an unsaturated gain of Go(vo) = 1.3 at the center of the Gaussian gain profile has a resonator length of J = 50 cm and total losses of 4%. Single-mode operation at vq is achieved with a coated tilted etalon inside the resonator. Design the optimum combination of etalon thickness and finesse. 

This results in a decrease and a broadening of the transmission maxima, which decreases the total finesse. If, for instance, a reflecting surface deviates by the amount Xjq from an ideal plane, the finesse cannot be larger than q. One can define the total finesse F of an interferometer by... 

This illustrates that high-quality optical surfaces are necessary to obtain ahigh total finesse [148]. It makes no sense to increase the reflectivity without... 

Analysis of the different pressings, as they come out of the press, shows significant variations in composition (Table 14.6). As pressing continues, total acidity decreases, as do both the tartaric and malic acid levels. Mineral content and pH increase, as do the phenolic content and color intensity, while the sugar level remains relatively constant. The aromatic intensity and finesse of wines made from successive pressings also diminishes, but we do not have the means to analyze these changes. 

Wedge finesse With a wedge angle of 0.2" the optical path between the two reflecting surfaces changes by about O.IA (A = 0.5/rm) across the diameter of the plate. This causes for a monochromatic, incident wave imperfect interference and broadens the maxima corresponding to a finesse of about 20. The total finesse is then F 2 = 1/(1/602+1/502+1/202) — F 17.7. 

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