Optical assembly and method for reducing zero-order light in a holographic display

A holographic display comprising an optical assembly, the optical assembly comprising:

• a light source;
• a light-modulation element arranged to be illuminated off-axis by a light beam and configured to modulate the incident light, the light-modulation element comprising an array of pixels and being arranged to generate a replay image and a zero-order light beam;
• a focusing system arranged after the light-modulation element and configured to focus modulated light from the light-modulation element and the zero-order light beam;
• a light remover positioned after the focusing system configured to remove the zero-order light beam from the light focused by the focusing system;
• a terminal optical element for directing modulated light from which the zero-order light has been removed by the light remover from the optical assembly;
• a collimator illuminated by a light beam from the light source to cause the light beam to converge;
• and
• a beam splitter configured to receive light from the collimator and reflect the received light onto the light-modulation element;
• wherein:
• an optical axis of the collimator is out of alignment with an optical axis of the beam splitter;
• the optical axis of the collimator is arranged at an angle with respect to the optical axis of the beam splitter such that a centre of the beam of light from the collimator hits the light-modulation element off-axis and in the centre of the light-modulation element;
• the angle is substantially equal to the value of the shortest wavelength of light in the light beam divided by twice the value of a pixel pitch of the array of pixels;
• all optical elements located after the light-modulation element, including the focusing system, are centred on a centre of the field of view of the replay image;
• the focusing system is configured to focus the zero-order light from the light-modulation element in a first plane different from a second plane which is the plane of focus of parallel light of the replay image, and
• the light remover is configured to remove the zero-order light in the first plane.