NOAO, in collaboration with Kaiser Optical Systems, Inc., has been evaluating a new grating technology with a grant funded by the National Science Foundation. Volume-phase holographic (VPH) gratings diffract light by means of index of refraction modulations within a volume of material. The light is diffracted by the classical diffraction equation, but the energy distribution is governed by the Bragg condition similar to X-ray diffraction by crystalline structures. For many potential applications, these gratings show superior performance characteristics over classically ruled gratings in efficiency and versatility. Previous discussions of this work have been given in earlier Newsletter articles (June 1998 and March 1999). We discuss further progress here.
Of the eight total volume-phase holographic (VPH) gratings to be fabricated for the study, seven have been fabricated and delivered to NOAO. Two of the gratings were previously discussed in the March 1999 article. Three more have now been evaluated and their performance is discussed below.
Grating HG-T-532-40 is a 2400 l/mm transmission grating designed for optimal efficiency at 532 nm. This grating actually peaks at 440 nm with an efficiency of nearly 89% (see Figure).
The reflection grating (HG-R-532-4/34) has a line frequency of 1200 l/mm and was designed to have a peak efficiency at 532 nm when illuminated at a grating angle of 4º. The preliminary measured efficiencies are quite high, peaking at about 90%.
VPH reflection gratings have a narrow bandwidth that may seem detrimental for its use in astronomical instruments, but there may be some interesting applications for which such a grating could have unique capabilities. For example, the undiffracted light passes straight through the grating and could be simultaneously utilized in either an imager or additional spectrograph without the need for a beam splitter.
One of the more interesting VPH gratings is a dual grating that diffracts
the light of H and H
to the same angle of diffraction (grating
HP-G-656/486-23). A 1200 l/mm grating was fabricated on one substrate and
mounted to a second substrate on which a 1620 l/mm grating was made. The
1200 l/mm component has peak diffraction efficiency at 656 nm and minimal
efficiency at 486 nm while the 1620 l/mm grating was designed to have
minimal efficiency at 656 nm and peak efficiency at 486 nm. A slight
rotation between the two gratings causes an angular deviation of the
spectra, so that they are well separated.
Except for one flaw, this grating fully lives up to its expectations and
meets the theoretical performance. The 1200 l/mm component has a peak
absolute efficiency of 93%! Unfortunately, the minimum in its diffraction
efficiency falls redward of 486 nm, so that this grating also diffracts
about 12% of the light at H instead of passing it all onto the 1620 l/mm
component. The 1620 l/mm grating, by itself, would nearly meet the
theoretical diffraction efficiency if some of the light were not diffracted
by the 1200 l/mm component.
An on-sky test of this grating was performed in late March at the 2.1-m telescope on Kitt Peak. The 200 µm fiber optic cable from the Fiber Optic Echelle was used to feed star light into a makeshift spectrograph, the QDS (Quick and Dirty Spectrograph) that was comprised of three achromatic lenses, the grating, and the T2KB CCD detector. The detector was binned into a 512 × 512 square format to better match the demagnification of the QDS setup. The fiber aperture subtended 2.5" on the sky.
The advantage of VPH gratings over classical ruled gratings is their
efficiency, a factor proven during this run by measurement of an overall
system efficiency of an amazing 29% for the H channel and 17% for H
. This
measurement included atmospheric absorption, efficiencies of the telescope,
spectrograph, grating, and detector, and seeing losses on the fiber
aperture. A spectrophotometric standard was observed with 1.2" seeing under
photometric conditions.
The two other gratings received, but not yet fully examined, are the 1064 nm, 2400 l/mm grating with prism substrates and the 532 nm, 4800 l/mm grating with prism substrates. Both have peak efficiencies of about 90% at their respective design wavelengths as measured by Kaiser Optical Systems, Inc. The last grating of the study, the 300 l/mm, high order grating, is under design and should be fabricated by the time you read this article. A future Newsletter article will cover the evaluation of these additional gratings.
To allow more direct access to the results of this NSF study, a web site (http://www.noao.edu/ets/vpgratings) has been developed at which further details of the grating evaluation can be examined along with links to other efforts involving the study, development, and use of VPH gratings.
Sam Barden