Digital light path length modulation systems

A modulation stack comprising:

• a polarization modulator to receive polarized light and to modulate a polarization of some or all of the polarized light;
• a waveplate;
• and
• an optical path length extender (OPLE) having an entry surface and an exit surface and a plurality of parallel polarization sensitive reflective elements, to direct the light entering the OPLE with a first polarization through the entry surface and along a first light path through the OPLE to exit through the exit surface, and to direct the light entering the OPLE with a second polarization through the entry surface and along a second light path through the OPLE to exit through the exit surface, the second light path through the OPLE having a light path length two or more times longer than the first light path length through the OPLE;
• blackout lines on the OPLE, along lines formed by polarization sensitive reflective elements, the blackout lines positioned on one or more of:
• the entry surface of the OPLE, and the exit surface of the OPLE.

A system to adjust light path length comprising a digital light path length modulator, the digital light path length modulator comprising:

• a polarization modulator to receive polarized light and to modulate a polarization of some or all of the polarized light;
• and
• an optical path length extender (OPLE) having an entry surface and an exit surface, the OPLE including a plurality of parallel polarization sensitive reflective elements to direct the light entering the OPLE with a first polarization through the entry surface and along a first light path through the OPLE to exit through the exit surface, and to direct the light entering the OPLE with a second polarization through the entry surface and along a second light path through the OPLE to exit through the exit surface, the second light path through the OPLE having a light path length two or more times longer than the first light path length through the OPLE;
• wherein each one of a plurality of OPLEs has a different configuration based on a variation of one or more of:
• a position of the polarization sensitive reflective elements, a spacing of the polarization sensitive reflective elements, a curvature of the OPLE, an unevenness in the entry surface of the OPLE, an unevenness in an exit surface of the OPLE, an angle of the entry surface of the OPLE, an angle of the exit surface of the OPLE;
• wherein the different configurations causes a different reflective pattern output by each of the plurality of OPLEs, which enables use of the OPLE as an optical key.