Intracavity frequency tuning has been demonstrated in many laser systems by using tilted birefringent plates which allow narrow band selection [1].In most common devices the resonator contains at least two pairs of surfaces oriented at Brewster's angle,those of the laser medium and the ones of the birefringent plate.In this cavity,the wavelength selection is achieved by rotating the birefringent plate in its own plane.However,the theoretical treatment of such devices,in relation with its bandwidth (FWHM for full width at half maximium) and working range (FSR for free spectral range),has been rarely worked out.Moreover,as far as we know,frequency tuning obtained by the laser medium itself has not been treated.The reason could be that only a few laser media are made from anisotropic crystals,and even in this case,only a few of them (mainly highly concentrated or stoichiometric laser crystals) can be used as birefringent gain plates in micro-laser cavities.On the other hand,frequency tuning in a birefringent gain crystal by self-birefringence filtering would be of interest only in broadband emitting lasers.
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