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
φ φ
λ
+ = ( ) md sin sin , 1 i rm,
where ϕi is the angle of the incident beam, φr,m is the angle of the
diffracted beam of the m. order, d is the period of the grating, and λ
is the wavelength. The wave, whose wavelength satisfies the
condition
λ φ G = 2 sin d G ( ) 2
is selected by the grating and is amplified. The other modes are
eliminated by focusing outside of the active region.