Virtually imaged phase arrays (VIPAs) (Figure 1) are a special kind of Fabry-Perot etalon composed of three unique coatings. An anti-reflection coated section represents a surface of a VIPA and is placed adjacent to a high reflector. A partially transmitting mirror is applied as a coating on the opposite surface. On the AR coated area, light is allowed to enter at a small angle into the VIPA. By titling the VIPA, the portion that is reflected off the partial reflector is now completely incident on the input surface’s high reflectance zone (Figures 2 and 3).
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Figure 1. Virtually imaged phase array – VIPA
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Figure 2. Beam enters via the AR coated entrance prism on top then grazes the edge of the high reflector coating
A single input beam is transformed into a series of parallel output beams with gradually diminishing intensity. These output beams will interfere constructively at an angle based on the wavelength. A spectrum of the input light can be recorded by positioning a lens between an array detector, such as CCD or other similar devices and the VIPA. Each successive beam has an accurate increase in phase and constant lateral displacement, hence the name “phase array.”
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Figure 3. VIPA shown here from the entrance side looking into the entrance prism
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Figure 4. VIPA, notice the staircase reflections and the similarity to an echelle grating
Performance of VIPA
The performance of a VIPA can be defined by a number of parameters. Optical thickness is the first parameter, which is OPD=2ntcos(θ) for a solid etalon.
Where n =Refractive index; T = Thickness; θ = Angle from normal within the VIPA
The free spectral range (FSR) is determined to be roughly c/OPD, using the optical thickness data. The angular dispersion of the VIPA output will repeat when there is an increase in the input frequency or wavelength by 1 FSR every time, like a regular etalon. The output mirror’s reflectance is the second important parameter.
Principally, the resolving power of the VIPA will be increased by a higher reflectance mirror. LightMachinery has optimized the partial reflectivity for every wavelength range for optimizing finesse. Finesse more than 100 has been obtained for visible/NIR applications - wavelengths separated by 1/100th of the FSR can be differentiated.
The internal angle of the light traversing the VIPA is the third important parameter. The angular dispersion is increased by smaller angles, but a lower limit is put on this angle by a couple of factors. The first reflection obtained from the partial reflector should be fully incident on the high reflector, so a smaller angle is enabled by a narrower transition between the high reflector and the antireflection coating.
The transition width of LightMachinery’s VIPAs is only 2-3 µm, enabling the VIPA to be operated at a very small angle. This angle reduction enhances contrast and resolution when compared to what was earlier possible. The reflected beam’s lateral offset should be more than the input beam width and the coating transition width collectively. Typically, this condition is maximized when the beam waist is positioned where the input beam reflects first from the partial reflector.
It is essential to select the VIPA coatings to match the desired wavelength range. In addition, the substrate material used must also be transparent. Fused silica is used to make the VIPAs in the LightMachinery catalog. Bespoke designs with silicon or calcium fluoride enable operation further into the infra-red.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.