Passively Mode-locked Yb³⁺-doped Fiber Laser Based On Single-wall Carbon Nanotubes

Passively mode-locked Yb3+-doped fiber lasers based on single-wall carbonnanotubes saturable absorber, have the advantages of simple structure, extremelynarrow output pulse-width, good self-starting property, high beam quality and so on.They have potential application prospect in optical communications, fiber sensing,biological technology, physical technology, micro processing etc. This thesis studybased on single-wall carbon nanotubes saturable absorber passively mode-lockedYb3+-doped fiber lasers, theoretically and experimentally. The detail research workcan be classified as follows:1．Numerically analyze the mode-locking mechanics of the Yb3+-doped fiber laser.Derivation of the relationships between the output coupling ratio k, cavity lengthL, slope efficiency η, and output power, is separately researched under thesituation of ideally lossless and actually loss. It is found that when the cavitylength is constant, in case of ideal, η is increasing with the increasing of k andtends to a stable value. In case of actual, when k increases, η increases firstly, thenreaches a maximum value and decreases gradually. When k take a certain value, incase of actual, increasing L, η rapidly increases then keeps at a stable value.2．Introduce the space energy level and physical characteristics of single-wall carbonnanotubes. Analyze its linear and nonlinear optical absorption properties.Experimentally pretreat the single-wall carbon nanotubes in order to make opticalsaturable absorber film. Put the nanotube powder into the benzene dichloridesolvent, supersonic stir to make it become homogeneous organic solution.3．Experimentally research on the passively mode-locked Yb3+-doped fiber laser,which uses the single-wall carbon nanotube as saturable absorber. By using opticaldeposition method, single-wall carbon nanotubes are coated as a saturableabsorber film onto the fiber end, and connected into the Yb3+-doped fiber laserring cavity. A stable mode-locked pulse output occurs. The total cavity length is13.5m. The repetition rate is14.85MHz and the pulse signal-to-noise ratio is60dB. When the incident pumping power is increased to the maximum330mW, theaverage output power is measured to be about7.23mW. Turning off the pumpsource and then reopen, the mode-locked lasers can be returned to the previousstate. It has a good ability to boot from the start.4．A phenomenon of multi-wavelength mode locking is observed. Increasing the incident pumping power, stable mode-locking pulses are still obtained and multipeaks appear in their spectrum. The central wavelength locates around1035nmand the average wavelength space is about1.5nm. The number of peaks and theirspace can be changed by adjusting the PC. When the incident pumping power isincreased to the maximum330mW,6equally spaced wavelength peaks occurs.5．The self-mode-locking behavior of the Yb3+-doped fiber laser is researched.Without single-wall carbon nanotubes saturable absorber in the fiber laser, themode-locking pulses still occurs. This is attributed to the high dopingconcentration and too-long of the gain medium. A part of the un-pumped gainfiber is act as a saturable absorber in the cavity. The experimental research showthat shortening the gain fiber, the mode-locking behaviors not exist anymore.