Non-deviating prism with continuously variable dispersion

#This invention relates to optical systems and having continuously variable dispersion characteristics, and more particularly to an arrangement of glass and liquid prisms which exhibit continuously variable chromatic dispersion without deviation.

#Non-deviating prisms are known. Such prisms are also known as direct view prisms. These prisms produce a fixed quantity of chromatic dispersion determined by the prism's geometry and the chromatic dispersion properties of the prism materials. One example of a non-deviating prism arrangement is the Dove prism. This type of prism is frequently used as an image rotator in a variety of opto-mechanical systems. An image passing through a dove prism will be rotated at twice the angular rate of rotation of the dove prism itself. The rotated image passes through the dove prism without angular deviation at some nominal wavelength (usually the center of the spectral band of interest) but does experience chromatic dispersion over the spectral band (e.g. the visible spectrum).

#Chromatic dispersion is the separation of polychromatic light (i.e. white light) into its chromatic components (i.e. colors) by passage through a refractive element such as a prism. This is caused by the interrelationship between the index of refraction of the prism materials, the prism geometry and the frequency (or wavelength) of the light passing through the prism.

#FIG. 1 shows an isometric view of a dove prism of the prior art. This prism transmits light rays with zero deviation (i.e. the nominal ray is neither displaced nor deviated in angle by passage through the prism). The light rays are however, chromatically dispersed by a fixed amount by passage through the prism;

#FIGS. 2A and 2B shows a variable dispersion prism arrangement of prior art in cross-sectional views. This arrangement is known as a rotating wedge prism pair. The dispersion of this arrangement is varied by rotating one prism relative to the other. The chromatic dispersion produced is accompanied by considerable deviation of the nominal optical path (i.e. the angular deviation of the nominal ray is larger than the angular chromatic dispersion produced between two well separated wavelengths (e.g. the red and blue ends of the visible spectrum);

#FIGS. 3A and 3B shows a cross-sectional view of the glass-liquid prism according to the present invention, in two angular settings which correspond to the zero dispersion position and the maximum dispersion position respectively;

#FIG. 1 shows an isometric view of a dove prism 8 of the prior art, which can transmit light with zero deviation and fixed amount of chromatic dispersion. Light entering the prism 8 along its optical axis 9 and is preferably parallel or collimated. More particularly, dove prism 8 includes six surfaces, respectively surfaces 11-16. The lateral surfaces of the prism (i.e., surfaces 11-14) are parallel to optical axis 9. Lateral surface 11, is referred to as the hypotenuse face. Light transmitted by the prism is incident on the hypotenuse face at such a shallow angle as to totally internally reflected. Surfaces 15 and 16 are the input and output faces of the prism.

#The construction of the dove prism ensures that ray 10a representing a nominal color or wavelength is transmitted without displacement or angular deviation. An incident polychromatic ray 9 is transmitted by the prism with chromatic angular dispersion about the nominal ray path 10. The angular spread of the dispersed light associated with a spectral band is denoted by dispersion angle .theta.. Dispersion angle .theta. is fixed by the prism geometry, the material employed and the spectral bandwidth of the transmitted light. Rotation of the Dove prism about the optical axis will rotate a transmitted image but the deviation will remain zero and the dispersion angle will be unchanged.

#FIGS. 2A and 2B shows in respective cross-sectional views, different settings of a variable dispersion, non-zero deviation optical system 20 of the prior art. Optical system 20 is an arrangement for variably dispersing light which includes a first prism 21 having first and second optical surfaces 25 and 26. Surfaces 25 and 26 are tilted with respect to each other at a predetermined apex angle. Optical system 20 further includes a second prism 31 having third and fourth optical surfaces 35 and 36 which are tilted with respect to each other at a second predetermined angle. Second prism 31 is axially rotatable with respect to said first prism 21 along the same optic axis. Typically, prisms 21 and 31 are identical so they have the same apex angles and the same index of refraction and dispersion properties.