Tunable mode-locked fiber lasers with optical fiber grating devices

Abstract

Mode-locked ultrafast fiber lasers have grown tremendously over the past decade because of their appealing application prospects in widespread fields, such as fiber-optic telecommunications, biomedical research, and spectroscopy. Recently, tunable mode-locked fiber lasers have attracted particular attention with the emerging of broadband absorbers made in carbon nanomaterials, like carbon nanotubes (CNTs) and graphene, as well as the development of wavelength tuning technologies. In this dissertation, tunable mode-locking technologies based on optical fiber grating devices have been investigated. First of all, a special LPG filter is developed for a widely tunable mode-locked fiber laser. By sandwiching a phase-shifted LPG (PSLPG) between two LPGs of different periods, a W-shaped spectral filter was experimentally fabricated. Through adjusting the temperature of the W-shaped filter, the emission wavelength of the mode-locked laser was tuned in a range covering both C and L bands. Because of its properly designed broad pass-band, the technology enables shorter pulses free of extra grating dispersion. Moreover, the spectral shaping effect of the PSLPG was investigated to effectively suppress the Kelly sidebands. Apart from thermally spectrum-tunable LPG devices, an optofluidic tunable mode-locked fiber laser is demonstrated based on the refractive index (RI) sensitivity of LPG. Ultrafine wavelength tuning was enabled by integrating a PSLPG into a home-made microfluidic chip. By accurately adjusting the RI of fluid, the emission wavelength of the mode-locked laser was tuned continuously. Experimentally, stable bound solitons with different pulse separations were observed. Such a grating-integrated optofluidic device offers a promising platform to develop novel ultrafast fiber lasers for scientific and industrial applications. Besides, a high-energy soliton fiber laser is demonstrated by using a CFBG pair. The CFBG pair with overall group velocity dispersion (GVD) of ~5 ps/nm/km was used to increase the net cavity anomalous dispersion. Stable high-energy mode-locked solitons with the typical pulse duration of 8.05 ps were obtained. The maximum pulse energy of 0.21 nJ, which is higher than the limit of conventional soliton area theorem, was achieved without soliton splitting. Numerical simulations using the split-step Fourier method (SSFM) revealed that the soliton-splitting threshold was improved by the CFBGs. Moreover, tunable vector and scalar solitons have been demonstrated via incorporating a CFBG in a non-polarization-maintaining (PM) and a PM fiber laser oscillator, respectively. The CFBG was firmly mounted on the central line of a cantilever and acted as an all-fiber tunable filter. In the non-PM fiber oscillator, L-band tunable vector solitons were obtained by using a CFBG centered at the wavelength of around 1610 nm and a Bismuth-based Erbium-doped fiber. Through adjusting the cantilever, the wavelength of the vector solitons was tuned continuously, meanwhile the polarization-locked state sustained. Numerical simulations revealed that the FWM sidebands were correlated with the cavity birefringence. In the PM fiber oscillator, stable scalar solitons with a 99.5% degree of polarization (DOP) were obtained. By adjusting the CFBGs around 1530 and 1560 nm, the laser wavelength was tuned continuously in the C band, while the laser output maintained its polarization stably.