A theoretical and experimental study of the effect of loss on grating couplers

The effect of material absorption loss on grating-coupler performance is presented. In particular we examine grating couplers fabricated holographically in photoresist. It is shown that a total field decay constant α
T
must be used in place of the leaky wave contant α whenever loss is appreciable. This total rate of field decay α
T
can be obtained, to a good approximation, by adding an absorption loss factor α
abs
to the leaky wave constant α. The absorption rate α
abs
is calculated by considering a suitable equivalent uniform waveguide and the leaky wave constant α is obtained by neglecting all loss in the structure except that due to beam radiation. Guided wave attenuation is found to be due almost entirely to absorption in the residual resist layer left between the grating and the waveguide rather than to absorption in the grating teeth. Experiments demonstrate that grating couplers fabricated in Shipley AZ 1350B photoresist have measured properties in agreement with the lossy description described above.