VPH Tutorial and FAQ
What wavelength range can VPH gratings made in DCG cover?
- Kaiser manufactures VPH gratings from 350 to 1700 nm.
- The wavelength range of a VPH grating formed in dichromated gelatin (DCG) is dominated by the material’s natural absorption characteristics. High transmission is available from 400 to 2700 nm. Outside this range gratings are still possible but the throughput is reduced by the material absorption.
What size of VPH gratings are available?
- Kaiser produces gratings from approximately 10 mm to 124 mm along the non-dispersion axis. The dispersion axis size can expand from the non-dispersion axis size according to the cosine of angles of the recording beams in the holographic recording system.
- NOTE: watch this space for an update!
Does Kaiser build GRISMs?
- Yes and have been for more than 16 years!
- A GRISM is a grating with prisms attached to the entrance and/or exit faces.
- The UltraSpec™-C160 telecommunication product is a GRISM.
- Kaiser has offered a GRISM based module for laser-cleanup since 1992: the Holographic Laser Bandpass Filter (HLBF).
- The High Disperison Grating (HDG) module manufactured for Kaiser’s RamanRxn suite of Raman-based instruments is a GRISM.
- Custom GRISMs have been designed and delivered for use in spectrographs for telecommunication, analytical spectroscopy, and astronomical applications.
What is the process of building a VPH grating in DCG?
- Obtain substrate materials
- Apply anti-reflection (AR) coatings if required
- Coat substrate with photosensitive emulsion
- Dichromated gelatin (DCG) used due to superior performance capability
- Expose coated substrate in holographic recording system
- Chemically process the exposed film to enhance latent image
- Test hologram performance
- Harden the processed film
- Laminate with a protective glass cover
- Trim bonded assembly to final physical format
- Final test physical and optical parameters to verify performance to specification
- Package and deliver VPH optical element
How is the VPH Grating Recorded?
- Two collimated beams of highly coherent laser light, generally called Reference and Object beams, are directed to intersect and optically interfere with each other at a location in space.
- The resultant microscopic interference pattern of alternating bright and dark parallel planes is recorded by placing a photosensitive material in the form of a film at the location of intersection.
- The period or spacing between the parallel planes is determined by the angles of the Reference and Object beams relative to the film’s surface and the wavelength of the laser light.
- The recorded grating is then described by the well known grating equation:
- sin(aR)-sin(aO)= l / d