New designs of optical devices, particularly for adding or dropping a selected
wavelength or a group of wavelengths are disclosed. In one embodiment, an optical
filter is positioned a distance from a first lens to form an assembly with a mechanical
axis. The optical filter is configured at a selected wavelength and reflecting
light beams at wavelengths other than the selected wavelength and transmitting
a light beam at the selected wavelength. The distance is obtained with respect
to a reflection measurement of a light beam at a wavelength other than the selected
wavelength such that the reflection measurement is minimized. A second lens is
initially positioned towards the optical filter. Before being fixed with respect
to the assembly, the second lens is laterally shifted from the mechanical axis
of the assembly to collect with a minimum loss a light beam refracted from the
assembly. Such lateral shift also benefits when a light beam comes out from the
second lens.
