Research On Passive Mode-locked Laser Based On Fiber With Inner Waveguide Saturatable Absorber

A saturable absorber based on optical fibers with inner waveguide is proposed,which provides a new solution for the integration of materials and optical fibers.Its main advantage is that the hybrid structure of graded index multimode fiber combined with graphene successfully realizes the secondary compression of the pulse,which is beneficial to the mode-locked laser to achieve a pulse output with a shorter pulse width.A passively mode-locked fiber laser with a waveguide saturable absorber has successfully achieved femtosecond mode-locked pulse output.This saturable absorber has the advantages of high mechanical quality,stable operation and wide application range,which makes up for the defect of poor mechanical quality of nonlinear materials in the coupling method based on tapered optical fiber and D-shaped optical fiber.The integration of fiber optic components offers a whole new approach.The specific content is arranged as follows:(1)The research of hybrid structure saturable absorber device combining graded index multimode fiber and graphene was carried out.The microstructure is written in the tapered graded-index multimode fiber,which effectively eliminates the precise requirement for the length of the multimode fiber when the single-mode-multimodesingle-mode structure is used as a saturable absorber.The surface of the tapered multimode fiber is covered with a graphene film,and the evanescent field generated by the cladding interacts with the graphene material,further shortening the pulse width of the mode-locked pulse.A mode-locked pulse with a central wavelength of 1567.09 nm,a spectral width of 4.43 nm and a pulse width of 709 fs was successfully obtained.The device has the advantages of firmness and stable operation,and is an ideal device for generating mode-locked pulses.(2)A saturable absorber device with a graphene film coated on the surface of an optical fiber with a built-in waveguide was carried out.Using femtosecond laser micromachining technology,the waveguide is inscribed in the single-mode fiber,and the surface of the fiber is coated with a graphene film.The core propagation mode is guided by the first section of the waveguide to the cladding and interacts with the graphene film in the form of evanescent waves,and then collected by the second section of the waveguide and returned to the core for transmission.The center wavelength of the output pulse of the constructed mode-locked laser is 1556.9 nm,and the pulse width is 830.5 fs.Compared with tapered fibers and side-polished fibers,the device is more stable and has high potential for high-power mode-locked laser pulse generation.(3)The prepared saturable absorber was connected to a linear cavity mode-locked laser,and a mode-locked pulse with a pulse width of 758 fs and a central wavelength of 1564.9 nm was obtained.The device effectively overcomes the shortcomings of tapered fibers and D-type fibers,provides a new optical coupling scheme for materialbased mode-locked lasers,and is expected to have good application value in low-cost,stable ultrafast lasers.