Liquid crystal tuned birefringent filter

A tunable optical filter comprising;

• a plurality of polarizers spaced at intervals along a propagation axis, each polarizer being characterized by a plane of polarization;
• a plurality of birefringent elements spaced along the propagation axis within the intervals between the polarizers, any two birefringent elements being separated by at least one polarizer, each birefringement element having an optic axis oriented so that the plane defined by the optic axis and the propagation axis forms a 45.degree. angle with the plane of polarization of each of the two polarizers immediately adjacent to and on opposite sides of that birefringent element, and at least one of the birefringent elements comprising a liquid crystal cell whose optic axis changes in response to a voltage applied to the cell;
• means for controllably tuning the filter to a selected wavelength by applying to each liquid crystal cell a voltage corresponding to the selected wavelength so that each birefringent element will produce a relative phase shift equal to a multiple of 180.degree. between the ordinary and extraordinary orthogonally polarized components of a light ray having the selected wavelength propagating along the propagation axis;
• wherein each birefringent element is characterized as operating at a particular order, the order of an element being defined as the quotient obtained by dividing 360.degree. into said relative phase shift equal to a multiple of 180.degree.;
• and
• wherein said filter includes an arithmetic progression of low order elements and a near geometric progression of high order elements.

A two-wavelength optical instrument comprising:

• first and second tunable optical filters, each filter comprising:
• a plurality of polarizers spaced at intervals along a propagation axis, each polarizer being characterized by a plane of polarization;
• a plurality of birefringent elements spaced along the propagation axis within the intervals between the polarizers, any two birefringent elements being separated by at least one polarizer, each birefringent element having an optic axis oriented so that the plane defined by the optic axis and the propagation axis forms a 45.degree. angle with the plane of polarization of each of the two polarizers immediately adjacent to and on opposite sides of that birefringent element, and at least one of the birefringent elements comprising a liquid crystal cell whose optic axis changes in response to a voltage applied to the cell;
• and
• means for controllably tuning the filter to a selected wavelength by applying to each liquid crystal cell a voltage corresponding to the selected wavelength so that each birefringent element will produce a relative phase shift equal to a multiple of 180.degree. between the ordinary and extraordinary orthogonally polarized components of a light ray having the selected wavelength propagating along the propagation axis;
• and
• modulator means, comprising a liquid crystal cell, for alternately allowing light transmission along two paths including the first and second filters, respectively;
• wherein the respective tuning means of the first and second filters may be tuned to two different selected wavelengths.

A tunable optical filter comprising;

• a plurality of polarizers spaced at intervals along a propagation axis, each polarizer being characterized by a plane of polarization;
• a plurality of birefringent elements spaced along the propagation axis within the intervals between the polarizers, any two birefringent elements being separated by at least one polarizer, each birefringent element having an optic axis oriented so that the plane defined by the optic axis and the propagation axis forms a 45.degree. angle with the plane of polarization of each of the two polarizers immediately adjacent to and on opposite sides of that birefringent element, wherein the plurality of birefringent elements includes plural liquid crystal cells each of whose optic axis changes in response to a voltage applied to the cell;
• means for applying the same voltage to each liquid crystal cell and for controllably tuning the filter to a selected wavelength by adjusting the voltage as a function of the selected wavelength so that each liquid crystal cell produces a relative phase shift equal to a multiple of 180.degree. between the ordinary and extraordinary orthogonally polarized components of a light ray having the selected wavelength propagating along the propagation axis;
• wherein each liquid crystal cell is characterized as operating at a particular order, the order of a cell being defined as the quotient obtained by dividing 360.degree. into said relative phase shift produced by that cell equal to a multiple of 180.degree., the plural liquid crystal cells including cells of different orders;
• and
• wherein each liquid crystal cell has a thickness along the propagation axis such that the ratios between the thicknesses of the cells equal the ratios between the orders of the cells.