Fabry-Perot interference filter

Computed evaluation of the spectral profile during the deposition of a Fabry-Perot interference filter (according to E. Pelletier, 1981), e = optical thickness of the individual layers. 

Figure 2 Principles of Fabry-Perot interference filter. (Adapted from Grum F and Becherer RJ (1979) Radiometry. New York Academic Press.)...

A Fabry-Perot (FP) interferometer can be used in an optical sensing system to pass a specific part of the spectrum, functioning like a monochromator. An FP structure is the most suitable for passband optical filters. A Fabry-Perot optical filter consists of two parallel mirrors with a resonance cavity in the middle (Fig. 2). The equation 7u = 2nd shows its operation principle, where n is the refractive index of the cavity medium, d the cavity length, X the incident wavelength, and q the interference order q = 1,2,3,...). 

Abstract This tutorial shows how fundamental is the role plaid by interferences in many of the physical processes involved in astrophysical signal formating and consequently instmmentation. It is obvious in interferometry. Grating spectroscopy is explained within the same framework as Young experiment, and Fabry-Perot filters are explained as Michelson interferometers.Polarization interferences, used in Lyot filters, are discussed, emphasizing the analogy with echelle gratings. 

Fabry-Perot interferometers, polarization interference, birefringence, Lyot filters... 

Tunable filters in the form of AOTF devices, liquid crystal tunable filter (LCTF) and also tunable cavity Fabry-Perot etalon (FPE) devices have been considered in non-moving part instrument designs for many years. Today, the AOTF and the LCTF devices are used in the NIR spectral region.9,10 Originally, designs were also proposed for mid-IR AOTF devices, but these have not become available, mainly because of fabrication issues (cost and material purity). Tunable FPE devices, which are really just variable cavity interference filters, have been developed for the telecommunications industry. While these have been primarily used in the NIR, in most cases they can be fabricated to work also in the mid-IR, the latter being only an issue of material/substrate selection. 

Interference filters are used in photometers and spectrometers as fixed wavelength or tunable wavelength filters. An interference filter, equivalent to a Fabry-Perot etalon. 

In Fabry-Perot etalons, the cavity encloses air, a gas, or vacuum, while for interference filters transparent dielectric layers are used. The real length of the interfering beam is N times the length of the cavity, due to multiple reflection. /V, the so-called finesse, is determined by the reflectivity p of the mirrors N = K , /pl( - p). Therefore, the resolving power, as above, equals the length of the interfering beam in units of the wavelength ... 

Figure 3.1-11 The significant features of Fabry-Perot-Etalons or interference line filters.

In an interference filter, a transparent dielectric spacing material separates two partially reflective windows. This conformation forms a Fabry-Perot filter, allowing a specific set of wavelengths to pass. The outer windows are constructed of materials with higher refractive index than the center spacer, which determines the central wavelength via its thickness. The equation describing the central wavelength is... 

Because the Fabry-Perot interferometer is a comb filter, an additional narrow bandpass interference filter is necessary to isolate the Brillouin scattering and reject Raman scattering or fluorescence. 

The most common and efficient type of bandpass filter is the interference filter. The basic principles of operation of such filters are illustrated by considering the simple Fabry-Perot filter. This is made by placing a spacer of dielectric medium between two partially transmitting metal films supported on a suitable substrate. These substrates are usually glass or quartz for the visible and ultraviolet whilst in the infrared they include germanium, silicon, IRTRAN II, IV and V, sapphire, indium arsenide, indium antimonide, and arsenic trisulphide. The basic principles of operation are shown in Figure 2. 

Fig. 4.56a,b. Interference filters of the Fabry-Perot type (a) with two single layers of silver (b) with dielectric multilayer coatings... 

Interference filters consist of a solid Fabry-Perot cavity. This is a device made of a sandwich of two partially reflective metallic layers separated by a transparent dielectric spacer layer. The partially reflective layers are made of higher refractive index than the dielectric spacer layer and are X/4 in thickness, where A is the peak wavelength (wavelength of maximum transmission) for the filter. The lower refractive index spacer layer is made to A/2 thickness. The thickness of the dielectric spacer layer determines the actual peak transmission wavelength for the filter. Only the A/2 light transmits with high efficiency the other wavelengths experience constructive interference between the multiple-order reflections from the two partially reflective layers. 

The wavelength position of the transmittance peak (A,) through either a Fabry-Perot interferometer or bandpass interference filter is given as... 

Two types of filters are used for wavelength selection interference filters, which are sometimes called Fabry-Perot fitters, and absorption filters. Absorption filters are restricted to the visible region of the spectrum interference filters, on the other hand, are available for the ultraviolet, visible, and well into the infrared region. 

Because of small imperfections and the finite size of the circular aperture the practically achievable resolution is frequently reduced. Because of its high resolution the Fabry-Perot interferometer has been much used for measuring hyperfine structure and isotope shifts. The spectral line of interest is then first selected by a monochromator or an interference filter. First, a free spectral range of sufficient width to accomodate all the spectral components of the line is chosen to allow the correct order of components to be determined. Then the plates are moved fiurther apart, resulting in an increased resolution but also the mclusion of overlapping orders. 

The importance of these materials goes beyond that of the design of broad-band transmission filters. The very same substances are also used in the constmction of interference filters, prisms, Fabry-Perot etalons, beam dividers, dichroic mirrors, and sometimes as windows to seal parts of an instrument while permitting radiation to pass. 

In Subsection 5.6.a we review the theory of multilayer thin films. In Subsection 5.6.b this theory is applied to the design of antireflection coatings for infrared windows, lenses, and other components. The same theory is used again in Section 5.6.C to find suitable beam dividers of the free-standing, self-supporting type as well as of the type requiring a transparent substrate. Subsection 5.6.d deals with interference filters and Fabry-Perot interferometers. 

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