LAUSR

Abstract A novel nonlinear optical switch based on a single mode fiber – graded index multimode fiber (GIMF) – single mode fiber (SMS) twined into polarization controller (PC) was proposed as a saturable absorber (SA), to generate ultrashort pulse in fiber laser. The SA device geometry, where a sandwiched GIMF section spliced to two standard SMF at both ends of GIMF, was coiled into a PC. By properly tuning the paddles of PC, mode-locking can be easily realized and the nonlinear saturable absorption characteristics of SMS-SA was changed. In the experiment, the modulation depth of the SMS-SA containing 0.6 m GIMF can be observed 9%, 10.5%, 21.7%, respectively. Based on SMS-SA mode-locking, the stable ultrafast pulse duration of 559 fs, with the FWHM bandwidth of 6.84 nm were generated. The fundamental repetition rate of 25 MHz and wavelength-tuning range of 14 nm can be obtained by adjusting the paddles. The SMS-SA excellent tunable nonlinear optical properties and all-fiber structure have strong potential application in the field of ultrafast photonics.