C-band wavelength tunable mode-locking fiber laser based on CD-SMS structure

A wavelength-tuning passively mode-locked all-fiber laser based on cascaded dual single mode fiber—graded index multimode fiber—single mode fiber (CD-SMS) structure is proposed in this paper. The CD-SMS structure geometry, where two SMSs were fused together, and each of SMS was coined into the paddles of polarization controller (PC). By adjusting the orientation angle of the paddles, magnitude of the nonlinear phase shifts of the light can be controlled to realize mode-locking and filtering. Experimentally, mode locking was obtained based on the SMS structure with pulse duration of 930 fs at the central wavelength 1568 nm and FWHM bandwidth of 2.9 nm. The fundamental repetition rate obtained was 18.22 MHz. By inserting another SMS into the ring cavity, and make-up CD-SMS, the five main peaks of mode-locked pulses were tuned in the range of 1533–1573 nm was achieved by mechanically tuning the orientation angles of paddles of two PCs, the wavelength tuning range covers the whole C-band, reaching 40 nm. However, the tunable range is limited by gain bandwidth of erbium ion. The wavelength-tuning mode locked fiber laser based on CD-SMS provides a way to a wide range of tunable ultrafast lasers.