SOIR - AOTF
An AOTF is a solid state optical filter that operates by the principle of acousto-optic diffraction in an anisotropic medium. The central wavelength of the filter passband can rapidly be tuned by changing the radio frequency (RF) of the signal applied to the TeO2 crystal’s transducer.
The choice of the AOTF bandwidth is critical: it should be less than the free spectral range (FSR) of the Echelle spectrometer, i.e. the spectral interval in which there is no interference or superposition of light from adjacent orders.
The acoustic wave generated inside the crystal at the transducer modifies the refractive index of the crystal which, in turn, causes the incident light beam to split into an undiffracted and two diffracted beams, one horizontally and the second one vertically polarized. SOIR uses a non-collinear AOTF.
Both the AOTF bandwidth and FSR are measured in terms of full width at half maximum. In the case of SOIR, the FSR equals 22.38 cm-1, and the bandwidth of the AOTF was originally designed to be 20 cm-1, as verified by measurement before launch.
The measured bandwidth of SOIR is ~24 cm-1 after launch, creating some order leakage overlap on the detector. The wavenumber domain that can be investigated by the SOIR instrument extends from 2256 to 4369 cm-1, and is divided into 94 smaller ranges corresponding to the different orders (from 101 to 194).
The detector width for orders 101 to 122 is smaller than the FSR of 22.38 cm-1 and hence the detector will lose part of the spectrum. For orders 123 to 194 the inverse happens: the detector width is equal to or larger than the FSR and the detector will not be completely covered by the selected order.
Description of the orders overlap occurring in the instrument due to the combined presence of the AOTF filter and the different diffraction orders of the echelle grating. In the Top panel, a simulated spectrum is shown containing only CO2. It spans several diffraction orders whose limits are also indicated. This spectrum is first filtered by the AOTF, and only the portion under the filter bandwidth enters the spectrometer. Due to the fact that this bandwidth is larger than the FSR of the echelle spectrometer, more than one order of diffraction are transmitted. This is illustrated in the Middle panel, where the contributions of the different orders are represented. Finally, the sum of all of these contributions is measured on the detector (Bottom panel)
In-flight performance and calibration of SPICAV SOIR onboard Venus Express
A new method for determining the transfer function of an Acousto Optical Tunable Filter