Performance of volume phase gratings manufactured using ultrafast laser inscription

1. Performance of volume phase gratings manufactured using ultrafast laser inscription David Lee & Colin Cunningham UK Astronomy Technology Centre Robert Thomson Heriot-Watt University

2. Introduction • Project overview. • Summary of the ultrafast laser inscription process. • Brief comparison of grating technology. • Fused silica prototype grating results. • Gallium Lanthanum Sulphide grating prototype results. • Summary and Conclusions.

3. Project aims • Capability of Ultrafast Laser Inscription (ULI) for manufacturing photonics components becoming well understood. • Aim of project is to investigate the potential of ULI technology to manufacture volume phase gratings for use in spectrometers. • Collaboration between STFC and Heriot-Watt University. • ULI technology likely to be particularly useful in mid-infrared.

4. Ultrafast laser inscription process • High peak power laser with femtosecond pulses. • Beam brought to a focus within the glass substrate. • Refractive index is permanently modified near the focus. • Translate glass block to inscribe 3D structures.

5. ULI manufacturing equipment • Ultrafast Ytterbrium doped fibre laser. • 340 fs pulses. • 500 kHz pulse rate. • Wavelength 1047 nm. • NA 0.4 objective. • Material for processing mounted on air bearing stages.

6. Ultrafast laser inscription • Ultrafast laser inscription can be used to manufacture a variety of optical and photonics structures • Waveguides • Fibre couplers • Fibre fan-out devices • Diffractive lenses • Bragg gratings • Volume phase gratings Example ULI fabricated integrated photonic lantern for multimode-to single mode conversion [R. R. Thomson, et al, Opt. Express 19, 5698-5705 (2011)]

7. Materials • Ultrafast laser inscription works well in a number of materials

8. Grating technology summary

9. Astronomical grating technology

10. Fused Silica ULI grating

11. Gallium Lanthanum Sulphide ULI grating GLS transmittance data from ChG Southampton

12. Efficiency measurement • Grating efficiency measured at 633 nm. • Stable Helium-Neon laser light source. • Grating on X-Y-rotation mount. • Beam intensity measured with an integrating sphere and photo-diode. Integrating sphere Laser

13. GLS grating efficiency • Maximum efficiency occurs at blaze angle of ~6 degrees. • Absolute efficiency includes the substrate transmission losses. • Efficiency of the grating alone is ~71%.

14. Fused Silica grating Scattered light performance Laser beam illuminates 1 grating. Scattered light within substrate causes the other gratings to glow. Halo of Scattered light -1 0 +1 +2 +3 Laser beam 633 nm Scattered light

15. Gallium Lanthanum Sulphide grating Scattered light performance Laser beam illuminates 1 grating. No scattered light within substrate, gratings are optically transparent. Laser beam illuminates 1 grating. Scattered light within substrate causes the other gratings to glow. Halo of Scattered light Scattered light between orders -2 -1 0 +1 -1 +2 0 +1 +2 +3 +3 Laser beam 633 nm Laser beam 633 nm Scattered light

16. Grating performance

17. Future work • Further optimisation of ULI process to reduce scattered light. • Better understanding of inter-order scattered light. • More robust characterisation of refractive index modulation. • Investigate other materials. • Manufacture of larger prototypes for on sky testing • Novel grating structures e.g. curved lines.

18. Summary and conclusions • This project has successfully demonstrated the use of ULI to manufacture volume phase gratings. • ULI can be used in a variety of materials. • The GLS gratings show promising performance and can be used over a broad range of wavelengths. • Technology potentially well suited to processing small, low line density gratings, for use at mid and far-infrared wavelengths.

19. Contact information • Thank you for listening. • David Lee: • David.Lee@stfc.ac.uk • Robert Thomson: • R.R.Thomson@hw.ac.uk