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 first 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