VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL SPECTROGRAPHS

1. VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL SPECTROGRAPHS James A. Arns, Willis S. Colburn, & Mark Benson (Kaiser Optical Systems, Inc.) Samuel C. Barden & Joel B. Williams (National Optical Astronomy Observatories*) * Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.

2. VPH Grating Physics(true holographic gratings) • Diffraction due to modulations in refractive index (Dn) of the grating material rather than by surface structure. • Light is diffracted at angles according to the classical diffraction equation: mnl = sin(a) - sin(b) • The energy distribution is governed by the Bragg condition: e.g. for a Littrow grating configuration - mnl = 2sin(a) • Grating depth (d) and index modulation (Dn) define Bragg performance.

3. Theoretical Diffraction Efficiency (h) • Rigorous Coupled Wave Analysis (RCWA) is typically required to model the effect of d and Dn on the grating efficiency (h). • Approximate theories give closed form solutions: e.g. in transmission gratings - RCWA efficiency for 3 transmis- sion gratings in 1st order. Where ag and bgare in the grating volume.

4. Bragg Envelopes Angular Bragg envelope. DaFWHM ~ L/d Spectral Bragg envelope. DlFWHM / l ~ (L/d)cot(ag)

5. VP Grating Configurations A.Littrow transmission configuration. B. Non-Littrow transmission configuration. C. Non-dispersive reflection (notch filter). D. Reflection grating configuration.

6. VP Grating Structures • Grating material: Dichromated Gelatin • Grating substrates: BK7, Fused Silica, etc. • Anti-reflective coatings on substrate-air surfaces. • Encapsulated nature protects gelatin from environment over a range in temperature and humidity. • Encapsulated nature allows surfaces to be cleaned without risk to grating. • Lifetimes of at least 20 years if properly handled.

7. Typical VP Grating Parameters • Line density: 300 to 6000 l/mm • Index modulation (Dn): 0.02 to 0.10 • Ave. index (n): 1.5 • Grating depth (d): 4 to 30 mm • Wavelength range: 0.4 to 1.5 mm • may be viable from 0.3 to 2.8 mm. • Grating size: 75 by 100 mm • limited by holographic exposure system • expandable to 500 by 700 mm

8. Dichromated Gelatin Transmittance of dichromated gelatin as a function of wavelength for a 15 mm thick layer which has been uniformly exposed and processed. Good transmittance covers the range from 0.3 to 2.8 mm.

9. Eight gratings are being fabricated and evaluated. 300 l/mm at 1064 nm 1200 l/mm at 532 nm 2400 l/mm at 532 nm 1200/1620 l/mm Ha/Hb multiplex grating 1200 l/mm reflection grating 2400 l/mm at 1064 nm with prism substrates 4800 l/mm at 532 nm with prism substrates 300 l/mm 10th order at 532 nm grating attempt • Gratings fabricated and evaluated. • Gratings fabricated. • Grating not yet fabricated. NSF Funded Study of VPH Gratings Some gratings will be distributed to US community at the end of this study.

10. 300 l/mm VPH Grating Measured absolute efficiency in unpolarized light for the 300 l/mm VPH grating. Note the tunability of this grating which shifts the blaze function to higher orders of diffraction as the grating angle is increased.

11. Measured efficiency in unpolarized light for the 1200 l/mm VPH grating. The solid line shows the efficiency at a grating angle of 19 deg. The dashed line shows the peak efficiency curve as the grating is tuned to different grating angles (the super blaze). The dotted line shows the efficiency for a theoretical 1200 l/mm surface-relief grating with similar blaze characteristics. 1200 l/mm VPH Grating

12. 2400 l/mm 532 nm VPH Grating Measured efficiency in unpolarized light for the 2400 l/mm VPH grating (optimized for 532 nm) at grating angles of 27, 33, 37, and 46 degrees. The super blaze shows the envelope of peak efficiency as the grating is tuned to different grating angles.

13. Ha/Hb Multiplex Grating A multiplex grating contains two gratings within one unit. The second grating operates near the minimum of the Bragg spectral bandwidth of the first grating. In this case, the 1200 l/mm grating diffracts Ha light while the 1620 l/mm grating component diffracts Hb light to the same angle of diffraction. Measured Efficiency for the 1200/1620 l/mm multiplex grating at grating angles of 17, 23, and 33 degrees.

14. Spectrum of 18th magnitude, blue compact galaxy obtained with the Ha/Hb multiplex grating with a fiber feed on the 2.1-meter telescope at Kitt Peak National Observatory.

15. RCWA Predicted Efficiency for VPH Reflection Grating This is the RCWA predicted efficiency for the 1200 l/mm VPH reflection grating. The bandwidths for reflection gratings are quite narrow. Wider bandwidths are possible by warping the fringe structure through processing techniques.

16. 1200 l/mm VPH Reflection Grating The diffraction efficiency of the 1200 l/mm reflection grating has not yet been directly measured. This plot shows the measured transmissivity of the grating at a tilt angle of 4 degrees. Except for absorption losses, the transmittance should be equal to the inverse of the diffraction efficiency.

17. Conclusions and Future Effort VPH grating technology will allow the fabrication of novel new instruments for astronomical spectroscopy. Several such instruments are already in the works at such facilities as the Anglo-Australian Observatory, NOAO (see related poster on NOAO’s spectrograph for the 4-meter telescopes), SOAR, and ESO. Future efforts will entail upgrading facilities to produce gratings as large as 200 to 300 mm (and possibly larger) and to continue to explore the unique capabilities of this technology. This project is supported under Cooperative Agreement AST-9613615 awarded by the National Science Foundation.