Study On Mode-locked Fiber Lasers Based On Gold Nanorods Saturable Absorbers And Their Applications

Mode-locked fiber lasers have been widely used in fiber communications, fibersensor, medicine, material processing and defense, due to the advantages of goodbeam quality, ultrashort pulse duration, simple configuration, compactness and lowcost. There are two main methods in realizing mode-locked laser operation. One isactive mode-locking in which electro-optic modulators are used in the laser cavity,which may cause the laser cavity became complexity and not all-fiber configuration.Another one is passive mode-locking, compared to active mode-locking, passivemode-locking has the advantages of simple configuration, good stability andultrashort pulse duration. A key element in the passive mode-locking fiber laser issaturable absorber (SA). Semiconductor SA mirrors (SESAMs) is the most commonSA for constructing passive mode-locking fiber lasers. However, it also requiresextra mirrors to focus the output light from fiber into bulk SAs. Therefore, SAs thathave good compatibility with optical fiber systems are essential for constructingall-fiber mode-locked lasers. Recently, carbon nanotubes, graphene, and topologicalisolators were used as SAs have been widely studied. They have the commonadvantages of good fiber compitability, ultrafast response time, large third ordernonlinear coefficient, and easy to manufacture.Gold nanomaterials SAs also attracted people’s attentions due to the advantagesof the large third-order nonlinear coefficient, good fiber compatibility, ultrafastresponse time, and easy to manufacture. In2012, our group first realized Q-switchedlaser at1.56μm wavelength by using gold nanoparticles as SA. In this work, goldnanoparticles only showed an absorption peak at523nm originate from transversesurface plasmon resonance (SPR) absorption. The operation wavelength of this kindof devices was limited. Broadband absorption from500-2000nm was caused by theaggregated gold nanoparticles in the film. However, it is difficult to control theaggregation of gold nanoparticles as required. During the doctoral years, the authorobtained a series of innovative research results of mode-locked fiber lasers based ongold nanorods (GNRs) SA and their applications. Details are as follws: 1. Based on our studies, compared to gold nanoparticles, GNRs have two surfacePlasmon resonance (SPR) absorption band. One is transverse SPR absorptionpeak at532nm and the other absorption band was caused by the longitudinalSPR. By varing the aspect ratios of GNRs, the longitudinal SPR absorption peakscan changed from visible to infrared wavelength. The broadband absorption ofGNRs indicates that it can be used to construct mode-locked or Q-switched fiberlasers, which operation wavelength covers from visible to infrared. Based on theabove findings, the author proposed that the longitudinal SPR absorption ofGNRs can be used as SA. By using this kind of SA, we first realizedmode-locked fiber laser at1.56μm experimentally. The pulse duration is about12ps with a repetition rate of34.7MHz and a maximum average power of2.05mW.2. We also studied on the relations between longitudinal SPR absorption peak andthe aspect ratios of GNRs. We found that the longitudinal SPR absorption peaksmoved to long waveleth with the increased aspect ratios, and also have saturableabsorption. By using GNRs4.5as SA, we realized mode-locking at1039nm inytterbium-doped fiber laser. The pulse duration is about440ps with a repetitionrate of36.6MHz and a maximum average power of1.25mW. By using GNRs5as SA, we realized mode-locking at1982nm in thulium-doped fiber laser. Thepulse duration is about4.02ps with a repetition rate of37.49MHz and amaximum average power of6mW.3. In addition,2or3μm mode-locking lasers can be obtained by using erbium orthulium doped fiber as gain medium. But at2-3μm region, there is no suitablerare-earth ions doped fibers. In order to solve this problem, the author proposedthat soliton self frequency shift (SSFS) effect can be used to construct widelytunable Raman soliton laser longer than2μm wavelength. In our work, werealized mode-locked fiber laser at2μm at first, and then scale up the averagepower by using chirp power amplified method. At last, the mode-locked laserwas injected into a piece of high nonlinear fiber, and then realized2-2.35μmwidely tunable Raman soliton laser by using the SSFS effect.