Abstract: A field effect transistor (FET) device including a GaAs substrate, an AlGaAs buffer layer provided on the substrate, an InGaAs channel layer provided on the buffer layer, an AlGaAs barrier layer provided on the channel layer, a GaAs undoped etch stop layer provided on the barrier layer where the undoped layer defines a depth of a gate recess in the FET device, and a heavily doped GaAs cap layer provided on the etch stop layer. The cap layer has a predetermined thickness and the thickness of the combination of the barrier layer and the undoped layer has the predetermined thickness, where the thickness of the undoped layer and the thickness of the barrier layer are selectively provided relative to each other so as to define the depth of the gate recess.

Abstract: The present invention deals with an optical transmission filter for effecting differential phase delay upon light in a beam polarized in a P dimension, as a function of position within the filter aperture for far field correction. The filter employs two lenses of birefrigent material, the crystal optic axes of the respective lens materials being oriented in mutually orthogonal positions and at a 45.degree. angle to the P dimension. The lenses have their adjacent surfaces respectively concave and convex with the same radius of curvature and their non-adjacent surfaces flat. The two lenses may be both spherical or both cylindrical.The correction provides an improvement in beam quality in laser systems operating with larger beam apertures or at higher powers.