How does your Volume Phase Holographic Grating (VPHG) technology differ from surface relief grating technology?

VPHGs, have a constant thickness (T) with a periodic, typically sinusoidal, change in refractive index (n). Surface relief gratings have a periodic change in thickness with a constant refractive index. Most VPHGs are made in transmission geometry while most surface relief gratings are made in reflection geometry. Surface relief gratings are copied mechanically, and thus both the surface relief grating original and copies will degrade over time as the number of copies increases.