Phase retarders normally show strong wavelength dependence. Achromatic retarders which exhibit nearly identical characteristics over a wide wavelength spectrum is used in polychromatic light. The present investigation deals with a technique to design and study the characteristics of an achromatic combination of birefringent plates in 800–2000 nm range. The retarder has been designed using calcite, crystalline quartz and ADP. The thicknesses of the plates are 19.38 𝜇m, 446.14 𝜇m and 12.57 𝜇m respectively. The new arrangement of three birefringent plates proposed has the promise of producing a zero-order quarter wave achromatic combination with fairly good accuracy.

In this communication, the construction and transmission characteristics of a narrow band-pass birefringent filter have been investigated. The filter is formed by a combination of two sets of retarders with a polariser in between, and two linear polarisers placed at the two extreme ends. The sets of retarders are chosen to be a cascaded fan-type Solc filter having two different kinds of retardations, resulting in filtering characteristics better than those birefringent filters proposed for the visible range. The performance of this proposed filter is better in terms of effective suppression of side peak amplitudes as well as reduction of bandwidth with less number of waveplates. This birefringent filter can be used inside a linear resonator.