Volume Phase Holographic Grating Evaluation - Progress Report

In the June 1998 issue of the NOAO Newsletter, we discussed our effort to evaluate the technology of volume-phase holographic (VPH) gratings which is being funded by the NSF. Much work has been accomplished over the past nine months on this project. To aid in this effort, we have hired Joel Williams (University of Arizona senior, majoring in Optical Sciences), as a laboratory assistant to carry out the grating measurements.

Of the eight NSF gratings to be fabricated, seven have either been fabricated or are in process at present (January 1999). Two have been fully evaluated: the 300 l/mm grating optimized for peak efficiency at 1064 nm, and the 1200 l/mm grating optimized for peak efficiency at 532 nm. Production of the final grating, a 300 l/mm, high-order diffraction grating, is on hold as we learn about the performance of the other gratings. Our understanding of those gratings will give us better insight to the design of this remaining grating.

The figures shown here give "blaze" profiles for the first two gratings in unpolarized light at different grating angles. The 300 l/mm grating (referred to as HG-T-1064-9) shows excellent efficiency in first and second orders of diffraction similar to that shown by the 600 l/mm grating discussed in the June 1998 article. It also shows good 3rd and 4th order efficiencies when appropriately tuned. The 1200 l/mm grating (HG-T-532-19) shows much higher efficiency than can be obtained with a comparable 1200 l/mm surface relief grating (which typically achieves only about 50% peak efficiency). The difference is due primarily to the less significant impact of polarization angle on diffraction efficiency for the VPH grating compared to the surface grating.

Caption: Measured Efficiency curves for the 300 l/mm VPH grating (HGT-1064-9) at grating angles of 6, 9, and 12 degrees.	Caption: Measured Efficiency curves for the 1200 l/mm VPH grating (HGT-532-19) at grating angles of 14, 21, and 28 degrees.

In other efforts, the VPH grism installed into the LDSS++ instrument at the Anglo-Australian Observatory was a great success, even though the grism was mistakenly fabricated at the incorrect line frequency. Please see the November 1998 AAO Newsletter at http://www.aao.gov.au/library/news.html for further details. Design studies are also underway at the AAO, SOAR, and NOAO for a variety of instruments, which would utilize this interesting technology. In addition, there is growing interest in the cryogenic behavior of these gratings for operation in the near-infrared spectral region. We intend to conduct some cryogenic testing in the near future.

Various results from the NSF grant will be presented at the "Imaging the Universe in Three Dimensions" conference at the end of March 1999; at the AAS summer meeting in Chicago; at the SPIE Annual Meeting in Denver in July 1999; and at the August 1999, Congress of the International Commission for Optics meeting in San Francisco. A variety of publications will also be generated.

Sam Barden