Curved volume phase holographic (VPH) diffraction grating with tilted fringes and spectrographs using same
The subject matter described herein includes a curved VPH grating with tilted fringes and spectrographs, both retroreflective and transmissive, that use such gratings. A VPH grating according to the subject matter described herein includes a first curved surface for receiving light to be diffracted. The grating includes an interior region having tilted fringes to diffract light that passes through the first surface. The grating further includes a second curved surface bounding the interior region on a side opposite the first surface and for passing light diffracted by the fringes.
1. A method for making a volume phase holographic diffraction grating, the method comprising:
placing a gelatin material between a mold and a substrate of desired curvature;
pressing the mold and substrate together to press the gelatin material into a holographic medium of desired thickness and curvature;
hardening the gelatin material; and
holographically encoding fringes in the gelatin material such that the fringes are tilted to diffract at least some of the light that passes through the gelatin material, wherein holographically encoding the fringes includes subjecting the gelatin material to first and second expanded laser beams and using a recording lens positioned in front of the grating, wherein the first and second laser beams strike the recording lens at different angles from each other, pass through the recording lens where they interfere with each other, are diverted and dispersed, and enter the grating to create fringes with tilt angles that vary from each other.
2. The method of claim 1 wherein holographically encoding the fringes includes subjecting the gelatin material to an interference pattern produced by interfering expanded laser beams that strike the grating symmetrically.
3. The method of claim 1 wherein the first and second laser beams are collimated.
4. The method of claim 1 wherein the first and second laser beams are noncollimated.
5. The method of claim 1 comprising placing shims between the mold and the substrate such that the holographic medium has the desired thickness.