Adaptive compensation of thermal lens in Faraday isolators

摘要

Two methods of compensation of thermal lensing in high wer terbium gallium garnet (TGG) Faraday isolators have been investigated in detail: compensation by means of an ordinary negative lens and compensation using FK51 Schott glass possessing a negative dn/dT. Key thermo-optic constants for TGG crystals and FK51 glass were measured. We find that the contribution of the photo-elastic effect to the total thermal lens cannot be neglected either for TGG or for FK51. We define a figure of merit for compensating glass and show that for FK51, an ordinary negative lens with an optimal focus is more efficient, but requires physical repositioning of the lens for different laser powers. In contrast, the use of FK51 as a compensating element is passive and works at any laser power, but is less effective than simple telescopic compensation. The efficiency of adaptive compensation can be considerably enhanced by using a compensating glass with figure of merit more than 50, a crystal with natural birefringence or gel.