OPTICAL COMPONENTS: SOI/MEMS blazed grating extends efficiency and tunability

Wavelength-tunable microelectromechanical systems (MEMS)-based gratings with blazed surfaces are not new; however, the high diffraction efficiency needed for telecommunications and spectroscopy applications is often obtained using complex and high-cost processes. Although tunable blazed gratings have previously been fabricated with surfaces at fixed angles of 54.7° using anisotropic silicon etching and at 7° using nanoimprint lithography, researchers at Tohoku University (Sendai, Japan) have now developed a tunable grating that uses a stair-step architecture to simulate a blazed surface, and more important, demonstrates improved first-order diffraction efficiency and extended tunability using silicon-on-insulator technology.¹

A theoretical treatment was first performed on the proposed grating structure to optimize the design. Calculations showed that for gold-coated, reflective silicon strips with a blazed grating approximated by a planar structure or a stair-stepped structure with two, four, or eight steps, structures with four steps approached the first-order diffraction performance efficiency of a truly blazed grating, and exhibited an 81.1% improvement in diffraction efficiency compared to planar gratings. Although an eight-step grating offered a 95% improvement, fabrication would be more complex.

The optical response of the four-step structure was simulated using rigorous coupled-wave analysis, which showed that the diffraction efficiency changes little even when the grating period is changed by using the microactuators to physically move the four-step silicon structures.

Fabricating stair steps

The grating structures are fabricated on a silicon-on-insulator (SOI) wafer with a 10 µm silicon device layer, a 1 µm buried oxide layer, and a 200 µm silicon handle layer. The four-step blazed surface profile is defined in a two-mask process, and the silicon is etched down to the buried oxide layer using reactive ion etching. Finally, a 5 nm/150 nm thick chromium/gold layer is deposited on the grating surfaces. The resultant gratings are 8 µm wide and 300 µm long, with 2 µm gaps on either side, for a 10 µm grating period. Gratings with different periodicity up to 20 µm were also fabricated for testing.

Opposing electrostatic comb-drive actuators are used to translate the free-standing grating structures for tunability. A pair of double-folded springs is used for each microactuator, and adjacent grating beams are connected by finger springs. After fabrication of the structures, a third mask patterns the microactuators and finger springs.

For a 20-µm-period device with 19 grating structures, applying 100 V to the microactuators can produce a 16 µm total shift in the overall grating, or a 0.84 µm elongation in the period of each grating structure that corresponds to a 65 nm tuning range for a device operating at 1550 nm. This approximate 4% tuning range is better than the 2.5% periodicity tuning achieved using planar gratings.

“This MEMS-based blazed grating will have a very high impact on industrial R&D on high-efficiency tunable gratings ranging from telecommunications to spectroscopic applications,” says Yongjin Wang, research fellow at the Japan Society for the Promotion of Science at Tohoku University. The gratings also can be used nanoelectromechanical systems for invisible and visible spectroscopy.

REFERENCE

1. Y. Wang et al., Optics Exp. 17(6) p. 4419 (March 16, 2009).