A review of the authors' work on orthogonal polarization of optical feedback in He–Ne and microchip Nd:YAG lasers is presented. Orthogonally polarized optical feedback has been applied to dual-frequency lasers and to monomode lasers with birefringent external cavity. The greatest advantage of the technique is that two modulated beams can be obtained with only one optical feedback path, so more information can be obtained than with conventional optical feedback. Polarization flipping with hysteresis and intensity transfer between the two eigenstates of the laser, the characteristics of intensity tuning, subdivision of the laser emission bandwidth, the phase relationship of the two orthogonally polarized intensity modulation curves, mode competition, signal frequency doubling through optical feedback, measurement of the small intracavity phase anisotropy in lasers, and antiphase intensity modulation of orthogonal polarization are experimentally and theoretically demonstrated. Possible applications are also discussed.
The influence of optical feedback on the longitudinal mode stability of microchip Nd:YAG lasers is demonstrated. Under optical feedback, the longitudinal mode output of microchip lasers relies strongly on the external cavity length, and mode suppression can be achieved. The intensity modulation waveform varies at different external cavity lengths. At higher feedback levels, the laser flips between orthogonal polarizations. Three important factors determining the influence of optical feedback are summarized, which are the external cavity length, longitudinal mode competition, and the external reflectivity.
We demonstrate a method of displacement measurement based on polarization hopping of laser with optical feedback. The measurement system is composed of a half-intracavity He-Ne laser, a quarter wave plate and an external feedback mirror. When the conditions of the second polarization flipping are satisfied, the frequency of intensity modulation can be doubled, and one polarization switching corresponds to displacement of external mirror. Meanwhile, the intensity transfer between two polarization states will come out, i.e., an increase of one polarization light intensity always accompanies a decrease of another polarization light intensity. One period of intensity transfer can be divided into four domains: e-light, e and o-light, o-light, and no light, and each domain corresponds to change of external cavity length. According to appearing sequence of the four domains, the movement direction of external mirror can be distinguished. Our method can improve the resolution of displacement measurement 8 times that of conventional optical feedback, and reaches 40nm for a laser wavelength of 632.8nm.