Fringe visibility

However, if the radiation is not monochromatic one has to consider each frequency separately and add the resulting fringe patterns, which have different separation (A. /b) between maxima. In this situation, the visibility fringes wash out , limiting the field of view of the interferometer. If the bandwidth is AX, the field of view is given by... 

As discussed in 2.3.2.2, it is possible to process CVD diamond plates to accuracy close to one visible fringe over 25 mm. This is equivalent to a deviation from flatness... 

The v sible wavelength beam splitter and compensating plate are both Yo (parallel to one second of arc) with 5 cm full aperture (Pyramid Optical Corp.). The beam splitter is located in a 45 optical mount (Burleigh) which is supported by a Star-Gimbal mount for adjustment. The fixed mirror is also the phase modulating mirror. In order to accomplish the phase modulation the mirror is mounted in a PZAT-90 (Burleigh) piezoelectric mount. There is sufficient linear motion available ( 10 micrometers/ 1000 V) from this mount to drive several orders of visible fringes. 

Partial reflections at the iimer optical interfaces of the interferometer lead to so-called secondary and tertiary fringe patterns as can be seen from figure B 1.20.4. These additional FECO patterns become clearly visible if the reflectivity of the silver mirrors is reduced. Methods for analysis of such secondary and tertiary FECO patterns were developed to extract infonnation about the topography of non-unifonn substrates [54]. 

The next source we investigate is the surface of an extended, limb-darkened star whose apparent diameter increases from 1 to 25 milli-arcseconds. The visibility function of such a source resembles the Airy function, varying periodically between zero and a maximum value which decreases with increasing frequency. Note how the fringe contrast vanishes repeatedly to rise again without reaching its previous maximum value as the source s apparent diameter... 

The fundamental quantity for interferometry is the source s visibility function. The spatial coherence properties of the source is connected with the two-dimensional Fourier transform of the spatial intensity distribution on the ce-setial sphere by virtue of the van Cittert - Zemike theorem. The measured fringe contrast is given by the source s visibility at a spatial frequency B/X, measured in units line pairs per radian. The temporal coherence properties is determined by the spectral distribution of the detected radiation. The measured fringe contrast therefore also depends on the spectral properties of the source and the instrument. 

The macrostructure of the boron nitride obtained here is porous with pores 2 pm in diameter. There is no evidence for microporosity and the BET surface area 1s 35 m2 g-1. Transmission electron micrographs (Figure 4) show regions of well developed crystallinity. The crystalling grains are 5—10 nm on a side and 30-40 nm long. The BN (002) lattice fringes are clearly visible. 

To evaluate its capability for refractive index measurement, the fiber FPI device was tested using various liquids including methanol, acetone, and 2-propanol at room temperature. The interference spectra of the device immersed in various liquids are shown in Fig. 7.12 for comparison. The signal intensity dropped when the device was immersed in liquids as a result of the reduced refractive index contrast and thus lowered Fresnel reflections from the cavity endfaces. However, the interference fringes maintained a similar visibility. The spectral distance between the two adjacent valleys also decreased, indicating the increase of refractive index of the medium inside the cavity. Using (7.4), the refractive indices of the liquids were calculated to be nmethanoi = 1 -3283, acetone = 1 -3577, and n2-propanoi = 1.3739, which was close to the commonly accepted values. 

These interferometers have been demonstrated for refractive index sensing. The sensitivity was around 33 nm spectral shift per RIU change when they were used for refractive index measurement. The maximum fringe visibility of these interferometers was around 9 dB, which was relatively small compared with the interferometers constructed by other methods. 

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