The light of a quasi-isotropic, single-mode gas laser is linearly polarized for a line J a = 1 -* J b = 2. However, the azimuth of polarization can vary inside the line. Among the many different effects which cause these variations, we describe the role played by the detector optical feedback. We show experimentally that its amplitude and phase can be varied in a controlled manner in order to obtain abrupt switches between two perpendicular polarization states. Polarization bistability and hysteresis effects can then be commanded and studied from the outside of the laser.PACS numbers: 42.55.Bi, 42.60.HeAs early as 1966, it was discovered 1,2 that a singlemode He-Ne laser with internal mirrors may operate with a polarization of light which can flip abruptly between two orthogonal states while the frequency is scanned. A great amount of work followed, showing first that atomic properties 3 "" 6 give a preferential polarization (circular or linear) and then describing the phenomenon both theoretically 7 " 10 and experimentally. 11 " 16 The subject has never been abandoned and now receives a renewal of interest in relation to laser instabilities, 17,18 optical bistability, 19 and vectorial tristability 20 studied in passive resonators. The commonly accepted idea is that the condition for bistable behavior in lasers is the presence of intracavity birefringence, generally attributed to the mirrors. We prove below that this requirement is not necessary, for this purpose we have divided the different causes of the polarization flip into three categories: (a) the properties of the gain medium, especially its anisotropy 21 or its longitudinal inhomogeneity of saturation, 22 (b) the cavity optical components, which are windows, 16 mirrors, 12 and diffracting objects 23 ; and (c) the anisotropic feedback of the outside.Up to now the third cause has been unseen in experimental studies. The new result in this Letter is the demonstration that one can command the polarization of light of a quasi-isotropic laser via the optical feedback of the detector system. The more isotropic the laser, the more sensitive it is to the feedback anisotropy. We study here a He-Ne laser working at 3.39 jxm where causes (a), (b), and (c) are in the following hierarchy: (a) is negligible, (b) is weak and controlled by the use of an amplifying tube which is closed by windows inclined 86° with respect to the laser axis, and (c) can be varied and increased enough to impose its anisotropy on the laser. We then obtain as many polarization flips as wanted in the emission line. Moreover, their positions are easily controlled. Applications include a new use of the laser as a means to determine the eigenvectors of an external device, the study of atomic properties via a measurement of regions of bistability, and an extension to multimode lasers in order to explain some controversial results. 24 Let us consider a monomode, internal-mirrors gas laser [ Fig. 1(a)] having a gain medium with a preferentially linearly polarized emission line. This is the case for ...
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