Achromatic holographic phase masks

A system, comprising:

• a light source that emits a collimated recording light beam at a recording wavelength;
• a beam splitter configured to split the recording light emitted from the light source into a first beam and a second beam, the first beam directed along a first beam path and the second beam directed along a second beam path;
• an optical device disposed in the first beam path, the optical device configured to induce a desired beam transformation for a broadband light beam that is to be transformed;
• at least one mirror configured to deflect at least one of the first and second light beams such that the first and second light beams converge at a recording angle;
• and
• a volume holographic recording medium having a photosensitivity to the recording wavelength and disposed at an intersection of the first and second beam paths, wherein
• the convergence of the first and second light beams records a spatial refractive index modulation profile in the volume holographic recording medium so as to create a volume Bragg grating (VBG) having a period such that a spectral selectivity of the VBG is at least as wide as a spectral width of the broadband light beam, and
• a phase profile is embedded in the VBG that induces the desired beam transformation for each spectral component within a spectral width of the VBG.

A method of hologram recording, comprising:

• selecting a period and thickness for a volume Bragg grating (VBG) such that a spectral selectivity of the VBG is at least as wide as a spectral width of a broadband light beam that is to be spatially transformed;
• selecting a desired beam transformation and corresponding phase profile across an aperture for the broadband light beam;
• selecting a recording light source emitting light at a recording wavelength that is within a photosensitivity spectrum of a volume holographic recording medium;
• designing and fabricating a phase mask or spatial light modulator that provides a desirable phase profile at a wavelength of a recording light source;
• splitting a beam from the recording light source into two recording beams;
• aligning a hologram recording system to provide a convergence angle between the two recording beams that provides the selected period of the VBG;
• passing a first recording light beam from the recording light source through a phase mask or a spatial light modulator to a volume holographic recording medium to create a first light beam path with the desirable phase profile across the aperture;
• directing a second recording light beam from the recording light source to the volume holographic recording medium to create a second light beam path;
• and
• converging the first light beam and the second beam at the convergence angle such that:
• a spatial refractive index modulation profile is recorded in the volume holographic recording medium that provides the VBG with the selected period, and
• a phase profile is embedded in the beam diffracted by the VBG that induces the desired beam transformation, which is identical for each spectral component of the broadband beam within the broadband spectral width of the VBG.