Research On Rectangular Noise-like Pulse In L-band Erbium-doped Fiber Laser

Noise-like pulses(NLPs)have the properties of high energy and low coherence,and are widely used in the fields of fiber sensing,laser micromachining and laser ablation.Fiber lasers base on silicon operates in L band have low loss and high transmission capacity.Therefore,L band lasers have been widely used in modern optical communication systems.Compared with conventional mode-locked pulses,rectangular noise-like pulses generated from fiber lasers based on nonlinear polarization rotation(NPR)technology is an effective way to obtain large energy pulses.To date,several fiber lasers operated in the conventional C-band(1530-1560nm)and L-band(1570-1620 nm)regions have been developed to deliver rectangular NLPs.The L-band erbium-doped fiber laser(EDFL)based on NPR technology has a compact all fiber structure and low mode-locked threshold,which provides a guarantee for the generation of stable and high-energy ultrashort pulses.In this thesis,a ring EDFL based on NPR is constructed and the dynamic behavior of rectangular NLPs in the laser cavity is studied.The main contents include:1.The research history of fiber laser is summarized,and the mode-locking principle of erbium-doped fiber laser is introduced.Meanwhile,the mode-locking mechanism of several common types of fiber lasers is explained.2.Nonlinear pulse transmission equation in optical fibers is studied.The pulse transmission equation in erbium-doped fiber laser is described.Using the Ginzburg-Landau equation,several mode-locked pulses,such as traditional soliton pulses and multi-pulses,are simulated and analyzed in EDFL.The research results are helpful to deepen the understanding of the mechanism and characteristics of laser mode-locking.3.The characteristics of L-band Q-switched and mode-locked rectangular noise-like pulse fiber lasers are studied.In the experiment,the traditional soliton mode-locking,rectangular noise-like pulse mode-locking and Q-switched mode-locking are realized at the central wavelength of 1600 nm(L-band)by adjusting the pump power and the polarization controller(PC)appropriately.When the pump power increases from 180 m W to 360 m W,the laser operates in a rectangular noise-like pulse region of Q-switched mode-locked laser.When the pump power increases to 380 m W,the working state of the laser changes to the noise-like mode-locked region.The research results are helpful to deepen the research on the dynamic characteristics of rectangular noise like mode-locked pulse and Q-switched mode-locked pulse.4.The characteristics of L-band multiple rectangular NLPs and harmonic pulses are studied.With the passive mode-locking function based on NPR technology,the laser is easy to achieve passive mode-locking.By properly adjusting the intracavity parameters(pump power and PC),the fundamental NLP mode-locking,harmonic NLP mode-locking and multiple rectangular NLPs are observed at the L-band with a central wavelength of 1570 nm.In harmonic NLP region,the 2nd to 5th order harmonics are observed.It is found that the pulse width of the rectangular pulses decrease with the increase of the number of harmonics.In the multiple rectangular NLPs region,the multiple rectangular NLPs with equal temporal seperation and unequal temporal seperation are observed.The observation results indicate that multiple NLPs not only have potential application prospect in molecular detection,remote sensing and laser communication,but also help people to understand the generation mechanism of NLPs in fiber laser system.