A relatively new grating technology shows exciting potential for improving the performance of the next generation of optical and near-IR astronomical spectrographs. Volume-phase holographic gratings can achieve higher diffractive efficiencies in many applications than currently available surface gratings. They may also lead to new spectrograph concepts that are currently impossible with surface gratings. NOAO has received an NSF grant to evaluate this grating technology for astronomy. Sam Barden will be collaborating on this study with Bill Colburn and Jim Arns of Kaiser Optical Systems, Inc. (KOSI) in Ann Arbor, Michigan. Eight gratings spanning a wide range of design parameters will be fabricated over the next year for evaluation. After the conclusion of the study, at least half of the gratings will be made available to the US astronomical community through the NSF.
Rather than having surface structure as in classical gratings, Volume-Phase (VP) gratings diffract light by refractive index modulations within a thin layer of material sandwiched between two glass substrates. The intensity of the refractive index modulation and the depth of the grating layer are critical parameters in the performance of the grating. Light is diffracted at angles corresponding to the classical grating equation as a function of the incident angle and the frequency of the index modulation at the surface of the grating. The diffraction efficiency, however, is a strong function of the relationship between the angle of incidence and angle of diffraction with respect to the fringes formed by the refractive index modulations within the volume of the grating. If these relationships satisfy the Bragg condition, which also depends on the depth of the grating volume and on the intensity of the grating fringes, then high peak diffraction efficiencies, approaching 100%, are possible. Good efficiency over moderately broad angular and spectral bandwidths can be achieved, but peak efficiency is usually decreased with increased bandwidth.
The first figure shows four possible grating configurations: A displays a transmission grating in which the index modulation fringes are normal to the grating surface; B shows a transmission grating in which the fringes are tilted with respect to the surface, resulting in a tilt of the Bragg condition with respect to the diffraction angles of the grating; C represents a reflective grating in which there is zero dispersion -- these are typically called notch filters; and D shows a reflection grating in which the fringes are tilted so that they intersect with the grating surface, resulting in dispersion of the diffracted light.
A transmissive 600 l/mm VP grating was acquired by NOAO to explore technologies for a high efficiency spectrograph. The grating was designed for peak diffraction at 700 nm with a bandwidth of 500 to 900 nm. The grating structure is that of Figure 1 A. Figure 2 shows the measured efficiency of the test VP grating in comparison with a comparable reflective surface relief grating in use at Kitt Peak. The VP grating also displays excellent diffraction performance outside the design criterion when it is tilted to angles that satisfy the Bragg condition for other wavelengths. Figure 3 shows the diffraction efficiency of the VP grating as a function of grating angle at the wavelength of 400 nm. Not only does the grating diffract efficiently when tuned for first order diffraction, but it also shows excellent efficiency for diffraction of 400 nm when tilted for 2nd and 3rd order diffraction. This "tunable" nature provides a versatility that is unmatched with classical surface gratings.
A simple, on-sky observation was obtained with a fiber feed at the 2.1-meter telescope on Kitt Peak. A standard star was observed in both first and second order configurations of the grating at 700 nm. Figure 4 displays the resultant spectra, in which the detected efficiency was identical in both configurations (17% total system efficiency, including sky, telescope, seeing, fiber transmission, collimating lens, grating, camera lens, and detector).
The tunable behavior of this grating was somewhat of a surprise to the makers at KOSI, as they had never examined the performance of their gratings outside the design envelope. There also appears to be minimal published research regarding higher order diffraction in VP gratings. A significant part of the NSF grant effort is focused on exploring the tunability, peak efficiency, and higher diffraction order performance of a variety of VP gratings.
The NSF grant will allow the fabrication of eight VP gratings. Final grating characteristics may change as we explore their design in detail, but the expected set of gratings will be:
1) 300 l/mm transmission grating optimized for first order at 1 Ám with a 500 nm bandwidth.
2) 1200 l/mm transmission grating optimized for first order diffraction at 600 nm with a 200 nm bandwidth.
3) 2400 l/mm transmission grating optimized for first order diffraction at 600 nm with a 60 nm bandwidth.
4) 2400 l/mm transmission grating optimized for first order diffraction at 1 micron with a 50 nm bandwidth.
5) 5000 l/mm transmission grating optimized for first order diffraction at 600 nm with a 25 nm bandwidth.
6) A dual transmissive VP grating structure designed so that 656 nm is diffracted by the first grating and 486 nm is diffracted at the same angle by the second grating. This is a complex grating structure in which the wavelength affected by one grating does not meet the Bragg condition for the other grating, so is only diffracted by one of the two gratings. This is a characteristic that is impossible to achieve with surface gratings.
7) 300 l/mm transmission grating optimized for tenth order diffraction at 600 nm with a bandwidth of 60 nm. This is a first attempt at a VP Echelle. It is not currently clear if the materials can provide adequate diffraction efficiency at such a high order.
8) 1200 l/mm reflection grating for first order diffraction at 600 nm with a 200 nm bandwidth. Although KOSI makes reflection holograms in their heads-up display combiners, they have not made a dispersive grating which works in reflection. This will be their first attempt at such a grating.
In addition to the NSF grant, Sam Barden is assisting KOSI in the implementation of a VP grism currently under fabrication for the LDSS spectrograph at the Anglo-Australian Observatory. A 400 l/mm grism with peak efficiency at 700 nm and a bandwidth of 500 to 1000 nm will serve as the dispersing element of the upgraded instrument with which the astronomers at the AAO plan to observe the Southern Hubble Deep Field with the AAT. Please refer to the January 1998, AAT Newsletter for more details.
Unfortunately, KOSI currently fabricates gratings only up to 75 mm in size. These are too small for astronomical spectrographs, which generally have beam sizes of at least 150 mm. However, there is an effort at KOSI to upgrade their holographic exposure system to make gratings at least 150 mm, and possibly 200 mm, in size. Their current expertise is in the fabrication of large holographic heads-up display combiners for the military, so the desired increase in grating size is within their range of experience in holographic fabrication. The current state of holographic technology is such that VP holographic gratings with dimensions of at least 600 mm are considered feasible. The availability of such grating sizes will be a requirement for the next generation of large telescopes and their spectrographs. For Further Information, please see the paper "Volume-phase holographic gratings and their potential for astronomical applications" by S. Barden, J. Arns, and B. Colburn, 1998, Proc. SPIE 3355 (NOAO Preprint No. 781) for details on the fundamentals of VP gratings.