The importance of passive mode locking of the fiber laser using the nonlinear Kerr effect has been stressed in recent years. In particular, passive mode locking of the all-fiber lasers using nonlinear fiber loop mirrors, such as the nonlinear optical loop mirror(NOLM) or the nonlinear amplifying loop mirror(NALM), has been shown to have a great potential for practical applications, e.g. high speed communication and fast optical switching.
In this paper, the characteristics of the NOLM, which is a key element for such passively mode-locked all-fiber lasers, were investigated. The reflectivity analysis of the NOLM by modeling the fiber loop as an elliptic retarder has given a good description about the polarization and spectral characteristics of the passively mode-locked pulses. The role that the loop birefringence plays in the passively mode-locked fiber laser system was understood through this study.
A stable and easy-starting passively mode-locked fiber laser, whose cavity was formed with a NALM at one end and a Faraday rotator mirror(FRM) at the other, was demonstrated. Our theoretical analysis based on the laser eigenmode theory showed several interesting features which explained how such stable and easy-starting passive mode locking was achieved when a FRM was inserted as one of the laser cavity mirrors. Our experimental observations have confirmed that, when a FRM was used in place of an ordinary planar mirror, passive mode locking started much more easily and passively mode-locked pulses were very stable. The width of the shortest pulse that we obtained was about 442fs assuming a Gaussian envelope, and the repetition rate was 4.3MHz which was equal to the inverse of one cavity round trip time.
A passively mode-locked figure-eight-laser(F8L) with a wavelength tunability and a harmonically mode-locked fiber laser with an all-fiber acousto-optic tunable filter(AOTF) were also briefly demonstrated. The tunable soliton F8L showed discrete tunability over t...