Fabrication of structures in an optical substrate

A method for fabricating structures in an optical substrate, the method comprising:

• producing first and second write beams from a first input beam and first and second reference beams from a second input beam;
• directing the first and second write beams to intersect at a first intersection location at the optical substrate, the first intersection location including a fringe pattern produced by the first and second write beams;
• directing the first and second reference beams to intersect at a second intersection location in substantially the same plane as the first intersection location;
• deriving a reference signal from the first and second reference beams at the second intersection location;
• and
• adjusting a relative positioning between the optical substrate and the first intersection location based on the reference signal.

A system for fabricating structures in an optical substrate, the system comprising:

• an optical assembly to produce a write input beam and a reference input beam from a light beam and to direct the write input beam and the reference input beam along substantially similar propagation paths;
• an interferometer to receive the write input beam and the reference input beam and produce first and second write beams from the write input beam and first and second reference beams from the reference input beam, the first and second write beams intersecting and producing a fringe pattern at a first intersection location at the optical substrate, the first and second reference beams intersecting at a second intersection location in substantially the same plane as the first intersection location;
• a beam combiner positioned at the second intersection location to recombine the first and second reference beams into a recombined beam;
• a detector positioned to receive the recombined beam and determine position information about the optical substrate and the fringe pattern based on the recombined beam;
• and
• a controller to control relative positioning between the optical substrate and the fringe pattern based on a signal derived from the recombined beam.