Optical depolarizer

An optical depolarizer comprising:

• an in-coupling polarizer for defining a linear polarization of an input optical beam at 45 degrees with respect to a first axis;
• a first walk-off crystal having first and second opposed ends,
• wherein the first end of the first walk-off crystal is coupled to the in-coupling polarizer, and
• wherein the first walk-off crystal is sized and oriented to split the input optical beam launched at its first end into first and second parallel laterally offset sub-beams exiting from its second end such that the first and second sub-beams at the second end are linearly polarized parallel and perpendicular, respectively, to the first axis;
• a slab of a transparent solid material, having a refractive index and a length between first and second opposed ends thereof,
• wherein the slab length multiplied by the slab refractive index is at least 1 mm, and
• wherein the slab is coupled at its first end to the second end of the first walk-off crystal and disposed in an optical path of the first sub-beam and not in an optical path of the second sub-beam,
• whereby in operation, when the input optical beam is launched into the in-coupling polarizer, the first sub-beam propagates in the slab from the first to the second end thereof, and the second sub-beam propagates proximate the slab in air, neutral gas, or vacuum;
• and
• a second walk-off crystal having first and second opposed ends, coupled at the first end thereof to the slab's second end and sized and oriented to recombine the first and second sub-beams at the second end of the second walk-off crystal into an output optical beam.