The Research On The Key Technology Of Mode-locked Fiber Laser

The generation of ultra-short pulses is not only the best source of ultra-fast, bigcapacitance, and long distance fiber communication system, but also the front edgeresearch about nonlinear optics and ultrafast optics. Fiber laser offer a number ofadvantages including small volume, light weight, compact structure, high stability, goodgain features, high conversion efficiency, low threshold, high beam quality andcompatibility with fiber-optic communication system. Now it has become one of thehottest topics in the field of the ultra-short pulse lasers. This paper takes Er³⁺ dopedpassively mode-locked fiber laser as the research keystone, made research on thefluorescence and side bands depressing methods, output ratio and polarization controloptimization, repetition rates increase technology, high energy pulse output, stability ofthe pulse and its use in UWB wireless communication, which include1. The advantages and disadvantages of actively mode-locking, passivelymode-locking and mixed mode-locking technologies have been analyzed. The physicalmechanism of mode-locking in the fiber laser was investigated. A novel model ofmode-locked fiber laser was developed by combining the rate equation of Er³⁺ dopedfiber and Nonlinear Schr(o|¨)dinger (NLS) equation. Investigation was made on thelongitudinal propagation characteristics of pump, mode-lock signal and AmplifiedSpontaneous Emission (ASE) power along longitudinal fiber with the proposed model,2. With the proposed mode and the emission mechanism of the fluorescence in highenergy pulsed fiber laser, the methods of effectively depressing the fluorescence andimprove the pump efficiency have been discussed. The Nonlinear PolarizationEvolution (NPE) passively mode-locked fiber laser experiments have been carried out.Mode-locked pulses with repetition rates of 15.9MHz, single pulse energy of 15.75pJand pulse width of 500fs have been obtained.3. Based on the comparing of numerical emulation and experiments of NPEpassively mode-locked fiber laser, the effects of different EDF lengths, different ringlengths and different output ratios on spectrum sidebands and pulse stability wereanalyzed. The ways of side bands depressing were obtained. Experiments with 9 kinds of output ratios on pulses quality were carried out, on which we summarized the ratioscorresponding to smaller fluctuation, larger single pulse energy, and wider spectrumwidth. And the single pulse energy would be the largest one (about 3.86nJ) whenR=70%. The output pulses with restrained side bands, the repetition rate of 15.87MHz,and the single pulse energy of 0.52nJ have been obtained.4. A new polarization control (PC) method was proposed based on Faraday opticsrotation mechanism in opto-magnetism crystal. This PC not only inherits theadvantages of electrical PC but also simplifies the complexity of circuits and save thecost of manufacture apparatus. The technology feasibility of this PC was experimentalcarried out, which prove that the polarization control ability of this PC is not linearitywith current intensity in the loop, the modulation ability of current intensity should beas precise as 0.01 A if we want to get the polarization control ability as precise as 1°.5. The ways of repetition rates control based on the modulation of pump power,output ratio and polarization controller were experimental carried out when the outputratios were high. The fiber laser spectrums and pulses in different harmonicmode-locking states were analyzed when the output ratios were between 40% and 90%.The compound bound pulse soliton states of fiber laser were also studied when theoutput ratios were 70%, 80% and 90%.6. A novel structure of passively mode-locked fiber laser was presented at highrepetition rates with an active slave ring cavity connected to a NPE master cavity.Based on the stable basic mode-locking state, the NPE mode-locked pulses wereinjected into the active slave cavity, and the ratio of master cavity length and slavecavity length was carefully modulated in order to achieve self-starting injectionmode-locking. Twenty-sevenfold evolution of repetition rates in NPE passivelymode-locking pulse was obtained at the basic mode-locking rank, from 12.9MHz to349MHz. And GHz rates were obtained from the slave cavity based on differentharmonic locking orders of the master cavity.7. A novel method for UWB (Ultra-Wide Band) microwave signal generation usinga passively mode-locked fiber laser is proposed and demonstrated. Er³⁺ deeply dopedfiber as long as 3m was used to get enough dispersion of fiber in order to make thepulse width wider to accord with FCC (US Federal Communications Commission)criterion about UWB wireless transportation in door, without using any high speed modulator or signal. Based on the principle of harmonic mode-locking and active slavecavity, 12 different kinds of UWB pulse periods were obtained.8. Experiments study of UW over fiber wireless transportation indoor was carriedout based on passively mode-locked fiber laser, technology of UWB optoelectronicconversion, UWB pulse amplifier and Bowtie aperture UWB antenna technology.Corresponding to amplified or not, gauss monocycle pulse (with FWHM of 150ps) ordoublet pulse (with FWHM of 120ps) was detected after UWB wireless transportationindoor. The influences of antennas distance, electromagnetic wave indoor disturbance,optical pulse source, attenuator and amplifier were studied in order to get the beststructure of this kind of system.