Fabrication And Exploration Of Echelle Grating With90°Apex Angle

Echelle gratings have a wide range of applications in spectroscopy, optical communications and other fields because of its ultra-high resolution. According to grating theory,90°apex angle is the first requirement of obtaining perfect blazing. Therefore,echelles with90°apex angle, comparing to non-90°, can achieve higher diffraction efficiency. Currently, mechanical ruling and wet etching technique are the main methods to fabricate echelle gratings. However, the fabrication of echelle gratings with90°apex angle still have many difficulties should to be resolved.Echelle gratings with90°apex angle have been successfully fabricated on a14°and35°off-cut (110) silicon wafer by anisotropic etch technique and inclined photolithography. The grating blazing surface still is (111) crystal plane, which is smooth and flat. And blaze angles are76°and55°。The diffraction efficiency of blazed orders was measured in a wavelength region of1500～1600nm. The results shows it agrees withcalculated efficiency based on C method. The grating has a strong blazing effect at wavelengths from1500to1550nm. And the measured diffraction efficiency is about52～75% of theoretical value, wherein the peak efficiency is about58%.For gratings with55°blaze angle, we compare diffraction efficiency of90°apex angle with70.53°apex angle. The results show that, diffraction efficiency of echelle grating with90°apex angle is3%-15%higher than that of70.53°apex angle.The reasons for the gap between measurement and calculated may include four factors:grating defects, grating spacing error, blaze angle error and groove depth error. The main effect of the first two factors is generating stray light. Andblaze angle error leads to the drifting of blaze order. The groove depth error is introduced during fabricating andhas a larger effect on diffraction efficiency. The grating’s groove depth error is about10%-15%. And the theoreticalefficiencyin consideration of groove depth error is about77%to85%of perfect groove efficiency at measurement band. Through the analysis of four factors, it points the improvement direction of fabrication process.