  We consider in this paper the angular deviation and diffraction efficiency of
the reflection gratings in Littrow-configuration for applications of external
cavity diode laser using the rigorous coupled-wave analysis method. We consider
the three-dimensional diffraction case in general, where the incidence plane is
un-parallel with the grating vector, i.e. conical diffraction. The angular
tolerance of arbitrary gratings under plane and conical diffraction are thus
derived and presented. A typical blazed grating is chosen as an example to
calculate its diffraction efficiency using the rigorous coupled-wave analysis
method. Furthermore, we point out that the angular tolerance and reflection
efficiency can be improved if the appropriate parameter settings are selected
for Littrow-configuration devices, including incidence angle, diffraction
order, grating period and blazed angle. Otherwise, a tiny slanting angle of the
grating vector deviated from the incidence plane will deviate the feedback
light away from entering the laser-diode-chip and halt laser oscillation in the
external cavity. Finally, a general rule for the parameter settings in
Littrow-configuration is provided as a benchmark.
