information on the other double-mode tunable laser types can be
found in, e.g. [1, 11 ] .In Fig. 1 , double-Littrow ECDL setup is shown. Such laser system consist of a gain chip, a collimator (lens or parabolic mirror), a beam splitter, and two wavelength selective elements (diffraction gratings or prisms). After collimating the laser beam from a gain chip using a lens, the beam is splitted in two parts utilizing a beam
splitter. The both beams reach then to the individual gratings. The incident beam and fi rst order of the diffracted beam have to be colinear for optimizing the optical feedback . The directions of the various orders of the diffracted beam with respect to the normal are determined by