Technical Information

TN2015-03: Virtually Imaged Phased Array (‘VIPA’) Etalons

When the creation of spatial dispersion is required, the most common solutions are to either use a prism or a diffraction grating. Another method is through the use of a modified solid or air-spaced etalon, a solution offering greater dispersive capabilities and insensitivity to polarisation. The modified etalon, shown in image TN-2015-03-1, is called a virtually imaged phased array (‘VIPA’) etalon. While a conventional Fabry-Perot interferometer has got two partially reflecting, matched coatings, a VIPA etalon has got a partially reflecting coating on one side while the other side is coated with a high reflective coating and a small entrance window that is either antireflection coated or has no coating at all.

VIPA Etalon -1
Image TN-2015-03-1: Conventional and VIPA etalons

A VIPA etalon is operated with the input beam at an angle to the normal of its surface. This specific way of inserting the beam allows the light to be reflected back and forth through the cavity. Tilting the VIPA etalon to vary the input angle of the beam is also a way of controlling the free spectral range.

The VIPA etalon will then convert the input beam into a series of parallel output beams of gradually reduced intensity. At an angle depending on the wavelength of the input beam the output beams will show constructive interference.

When a lens is placed between the VIPA etalon and a detector, the spectrum of the input beam can be determined. Each of the subsequent beams will show an exact increase in phase and displacement.

A VIPA etalon can also be air-spaced and all information given above applies similarly to air-spaced VIPA etalons.

In terms of advantages and disadvantages, the same applies to VIPA solid and air-spaced etalons as does to their conventional counterparts. Air-spaced etalons are much more thermally stable than solid etalons but solid etalons, on the other hand are much more compact. As solid etalons are essentially single plates, they are also more rugged than air-spaced etalons. As a result of this, there is also a thickness limit, both upper and lower, on solid etalons.

The upper thickness limit is dictated by the availability of substrate material with suitable homogeneity, while the lower thickness limit is dictated by manufacturing practicalities. MPO has a long history of pushing the boundaries of manufacturability. One of many examples are solid etalons with a size of 4mm x 4mm and a thickness of 19µm – thinner than a human hair.

When specifying an etalon, all manufacturing tolerances must be taken into account to ensure the final product meets the requirements. This is not always a simple task, but the experienced team at Manx Precision Optics is always on hand to help and discuss all specific aspects of the application to ensure the best solution can be offered.

For further information, please do not hesitate to contact us.

Manx Precision Optics Ltd.
4 Albert Street
Isle of Man IM1 2QA
British Isles

Tel: +44- (0) 1624-620 800