Tunable Fabry-Perot optical filter

Lay out a tunable Fabry-Perot optical filter in a multiproject wafer process as shown in Figure 4.24. The top Bragg mirror should be composed of a stack of dielectric films and should be flat to within a fraction of a wavelength of light that will be filtered. 

Receiving equipment for aircraft astrophysical investigations in the far infrared has been developed and flown in 1974-1975 on board the AN-30 aircraft. Four types of infrared and submillimeter detectors mainly of photoresistor type are used in the wavelength region from 10 mkm to 1 mm. Thermal background at the detectors is limited by cooled bandpass filters and by optimal matching of the detectors with the telescope optics. The spectral filters used are combinations of quasiresonance metal mesh filters of different structure and Q-factor from 2 to 7 with the Yamada cut-off powder filters. A tunable Fabry-Perot interferometer, a polarimeter for linear polarization measurements and an aircraft 25 cm-telescope are briefly described. 

D. Bonaccini, L. Casini, P. Stefanini. Fabry-Perot tunable filter for the visible and near IR using nematic liquid crystals. In Current Developments in Optical Engineering IV, Robert E. Fischer and Warren J. Smith, eds. SPIE—The International Society for Optical Engineering, San Diego, CA, 1990, 1334 221-230. 

Fabry-Perot etalon devices based on materials sensitive to electric fields have been studied as tuneable optical filters. Gan et al. [165] presented in their work a hybrid organic/inorganic sol-gel material based on 3-methacryloxypropyl trimethoxysilane as precursor and TCBD (15% mol) as NLO chromophore that was covalently incorporated into the silica network. They demonstrated an optical filter having large tunability and high finesse. The EO sol-gel film was corona poled with 5 kV/cm at 170°C for 30 min. 

There are two types of spectrometers using optical filters. The first type is equipped with a number of optical filters transmitting NIR light of different wavelengths, which are used sequentially to transmit spectroscopic data. The other type applies wavelength-tunable Alters such as a liquid crystal tunable filter (LCTF) or a Fabry-Perot interferometer. 

The distance between the mirrors can be adjusted with a MEMS actuator to enable a tunable interferometer. When the distance between the mirrors is adjusted to 2/2, the light passes through the interferometer while light with other wavelengths is blocked. The Fabry-Perot interferometer acts as a narrow bandpass filter for light, with the bandpass is dependent on the finesse of the optical cavity. 

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