Optical materials for near infrared Wollaston prisms

¹ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I–50125 Firenze, Italy
² Dipartimento di Astronomia e Scienza dell Spazio, Università di Firenze, Largo E. Fermi 5, I–50125 Firenze, Italy
³ Istituto Nazionale di Ottica, Largo E. Fermi 6, I–50125 Firenze, Italy

Send offprint request to: E. Oliva

Received: 14 May 1996
Accepted: 16 September 1996

Abstract

The optical characteristics of birefringent materials transmitting in the near IR (0.9-2.5 μm) are analyzed. Wollaston prisms with large beam separation and virtually free from lateral chromatism – e.g. with fields of view arcmin and image elongation – could be manufactured using  or  (YLF). These materials are used for non linear applications and may therefore find another interesting application in IR focal plane instruments. From the optical point of view the best material is  but YLF may be practically more convenient because of its lower refractive index (), better thermo–mechanical properties and lower price. New measurements of the transparency of Calcite at room and cryogenic temperatures are also presented, these show significant absorption ( ) of the ordinary ray already at μm and very strong bands (absorption coefficient cm^-1) beyond 2.25 μm. The absorption does not decrease significantly when the crystals is cooled,  should not be therefore used in instruments working beyond 2.0 μm.