VIPAs, Virtually Imaged Phased Arrays, are at the heart of many of LightMachinery's spectrometers. A VIPA is a specific type of Fabry-Perot etalon with three distinct coatings. One surface of a VIPA has an anti-reflection (AR) coated section next to a high reflector. The opposite surface is covered with a partially transmitting mirror.
Light enters the VIPA at a small angle via the AR coated region. Tilting the VIPA means the light reflected from the partial reflector is fully incident on the high-reflectance area of the input surface.
How VIPAs Work
A single input beam is turned into a number of parallel output beams with gradually diminishing intensities. Depending on their wavelengths, the beams interfere constructively at varying angles.
Placing a lens between the VIPA and an array detector (CCD or similar) captures the spectrum of the input light. Each consecutive beam has a precise increase in phase and a predetermined lateral displacement, described as a “phase array.”
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Performance Parameters
Multiple parameters define VIPA performance. The first is the optical thickness. For a solid etalon, OPD = 2ntcos(θ), where n is the refractive index, t is the thickness, and θ is the angle from normal inside the VIPA.
Based on the optical thickness, the free spectral range (FSR) is about c/OPD. The angular dispersion of the VIPA output, like that of a conventional etalon, repeats every time the input frequency (or wavelength) rises by one FSR.
Another key parameter is the output mirror reflectance and finesse. In principle, a higher reflectivity mirror will improve the VIPA's resolving capability. LightMachinery has optimized the partial reflectivity for each wavelength range to maximize finesse. For visible/NIR applications, finesse was over 100, meaning even wavelengths separated by one hundredth of the FSR will be distinguishable.
The third parameter to assess is the internal angle of light passing through the VIPA. Smaller angles enhance angular dispersion, although a few variables restrict this angle. The initial reflection from the partial reflector must be completely incident on the high reflector, and a thinner transition between the antireflection coating and the high reflector allows for a lower angle and more angular dispersion.
VIPAs can operate at small angles because of their narrow transition width of two to three micrometers. This decrease in angle enhances resolution and contrast above what was previously feasible. Second, the reflected beam's lateral offset must exceed the input beam's width plus the coating transition's width. This is typically enhanced by placing the beam waist where the input beam first reflects from the partial reflector.
Materials and Coatings
VIPA coatings should correspond to the wavelength range of interest, and the substrate material must be transparent. All LightMachinery catalog VIPAs are composed of fused silica. Customized designs using calcium fluoride or silicon enable operating in the infrared.
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