A QUANTITATIVE STUDY OF THE POLARIZATION STATES OF A QUASI-ISOTROPIC 3.39-MU-M HE-NE-LASER SUBJECT TO WEAK OPTICAL FEEDBACK

We present an extensive experimental study of the polarization behavio ur of a quasi-isotropic He-Ne gas laser operating in a single-transver se, single-longitudinal mode at 3.39 mu m, and subject to weak polariz ed optical feedback. We make a quantitative comparison of the experime ntal results with a vector extension of Lamb's single-mode laser theor y. We concern ourselves with both the qualitative and quantitative asp ects of instabilities and the resulting hysteresis loops in the polari zation state. In particular, the location of the instabilities and the width of the hysteresis loops within the emission line are examined a s a function of the experimental control parameters. The relationship between these instability scenarios and how the system moves catastrop hically from one kind of hysteresis loop to another is illustrated. Sp ecifically, we show that the polarization behaviour of the laser is co ntrollable by means of the weak polarized feedback from optical compon ents external to the cavity. The importance of the strength, orientati on and phase of the feedback is clearly demonstrated.