NEW FOCUSING OPTICS FOR 2ND-HARMONIC WAVES RADIATED FROM CRYSTAL-CORED FIBERS

The optical second-harmonic (SH) wave generated from a crystal-cored f iber under Cerenkov-radiation-type phase matching can be collimated by an axicon and then can be focused by a spherical lens to a diffractio n-limited spot. The deviation of the radiation angle of the SH wave or the angle mismatch between the radiation angle and apex angle of the axicon causes spherical aberration. However, because the focusing syst em using the axicon is sensitive, such a system is not practical enoug h. A new optical system is proposed that consists of three spherical l enses: biconvex, biconcave, and meniscus lenses. The system can collim ate the SH light radiated from the fiber. The new lens system is overc orrected for spherical aberration in certain zones. When the radiation angle of the SH wave deviates from the desired angle, the aberration can be compensated by adjusting the spacing between the biconcave lens and the meniscus lens.