We have experimentally demonstrated a high level of control of the mode populations of guided atom lasers (GALs) by showing that the entropy per particle of an optically GAL, and the one of the trapped Bose Einstein condensate (BEC) from which it has been produced are the same. The BEC is prepared in a crossed beam optical dipole trap. We have achieved isentropic outcoupling for both magnetic and optical schemes. We can prepare GAL in a nearly pure monomode regime (85 % in the ground state). Furthermore, optical outcoupling enables the production of spinor guided atom lasers and opens the possibility to tailor their polarization.Isentropic transformations have been used extensively to manipulate classical and degenerate quantum gases. For instance, the reversible formation of a molecular BEC from ultra-cold fermionic atoms having two spin components was performed by adiabatically tuning the interspecies scattering length from positive to negative values [1,2,3,4,5]. Another illustration is the adiabatic change of the shape of the confining trap of cold atoms giving the possibility to change the phase space density in a controlled manner [6,7]. A spectacular demonstration of this idea has been the multiple reversible formation of BEC by adiabatically superimposing an optical dimple trap on a magnetically trapped and pre-cooled sample of atoms [7]. For an ideal transformation, the entropy of the initial cloud should remain constant. However, the limitations of the experimental setup always introduce an extra source of entropy. The challenge for the experimentalists is to minimize this latter contribution.In this experiment, we demonstrate the control and the characterization of a guided atom laser (GAL) [8,9]. The experiments performed to date on the beam quality were addressing the spatial mode of free-falling atom lasers. The importance of the outcoupling scheme or the role played by atom-atom interactions has been extensively studied [10,11,12]. GALs are characterized by the population of the transverse modes of the guide. We have extended the use of isentropic analysis to propagating matter waves in order to relate quantitatively these populations to the characteristics of the BEC from which the GAL originates. This approach turns out to be possible because of both the validity of the local thermal equilibrium and the sufficiently large reduction of the extra entropy production generated by the experimental manipulation. Improvements to the production and characterization of the GAL are crucial for fundamental studies such as quantum transport [13], and applications in metrology [14], among others.The experiment starts by loading 3 × 10 7 87 Rb-atoms in a crossed beam optical dipole trap at a wavelength of 1070 nm from an elongated magneto-optical trap (MOT). To transfer the atoms in the lower hyperfine level F = 1, we align the horizontal arm (1) of the optical trap with the longŷ-axis of the MOT (see inset of Fig. 1.a) and we mask the repump light in the overlapping region between the two traps. The other ar...