Design And Preparation Of Graphene Saturable Absorber Mirror And Studies Of Mode-locking

In recent years,with the rapid emerging of mobile Internet,cloud computing,big data,Internet of Things and other services,traffic surge has brought great challenges to the communication network.The effective way to solve the problem of explosive growth of network data flow is multiplexing technology,especially the OTDM and DWDM,which can expand the capacity of optical fiber communication system.Ultrashort pulse lasers precisely meet the needs of the two multiplexing technologies,and can overcome the speed restriction of electronic devices from ETDM and the quantity limitation of multiple light sources from DWDM.Ultrashort pulse lasers provide an important tool to probe the dynamics of physical systems at very short timescales and also open even more applications such as medical cutting,material processing and metrology.At present,the dominant technology of ultrashort pulse lasers is based on III-V QW or QD semiconductor saturable absorber mirrors(SESAMs).However,these have a narrow tuning range(tens of nanometers),low power threshold,and require complex fabrication and packaging.In order to match absorption wavelength,changing the proportion of semiconductor material composition is necessary,which results in a significant lattice mismatch when above 1.3?m.This lattice mismatch reduces the quality,resulting in higher insertion losses,and reduce laser power.Graphene has become a hot spot of research,since its discovery in 2004.Its electronic and mechanical properties are also ideal for micro-and nanomechanical systems,thin-film transistors,and transparent and conductive composites and electrodes.In addition,pristine graphene acts as fast absorber and is considered to be an efficient mode-locking material for ultrafast lasers.Graphene as a laser mode locker can have many merits such a ultra wide absorption spectral range,low saturation energy fluence,ultrafast recovery time,and high thermal damage threshold.In this paper,we mainly demonstrate the design,preparation and experiments of a graphene saturable absorber mirror(GSAM)at 1064nm and 1550nm.The majorwork of this paperis as follows:1.First of all,we theoretically study of the mode locking technology,and then brief advantages of passively mode-locking compared with actively clamping and self-locking.This paper systematically analyzes fundamentals of traditional ?-?SESAMs,including Bragg mirror,saturable absorber,an antiresonance.Moreover,we investigate the influence of SESAM's macro-parameters on mode locking,the transition between the regimes of cw mode locking and Q-switched mode locking,suppression of Q-switching instabilities,and the advantages and disadvantages of several kinds of SESAM structure.2.We investigate the unique band structure and saturable absorption of graphene,and analyze the universal linear optical absorption and ultra-fast dynamics process.We design the structure of GSAM both for 1064 and 1550nm,and discuss the appropriate the number of layers,when graphene as saturable absorber.3.Graphene is grown on Cu foil by chemical vapour deposition(CVD)and then transferred onto the gold-film substrate.Since the real operation bandwidth of graphene saturable absorber is usually limited by the bandwidth of the substrate,we simulate reflection spectrum of the gold mirror for different thickness.We measure the reflection spectrum of GaAs-Cr-Au for the gold film being 100nm and 200nm.The experimental results show that the GaAs-Cr-Au has a high reflectivity ranging from 900 nm to 1800 nm.Furthermore,we measure the Raman spectrum of the there-layer graphene.The D peak,which is usually observed in defective graphene,is almost suppressed.In the session of adjustment,the Er:Yb:YAB laser crystal at 1550 nm is broken,we have to only test Nd3+:GdVO4 bulk laser at 1064nm.Because modulation depth is larger with multiple-layer graphene,With monolayer graphene,it is difficult to initiate the mode locking.Finally,with three-layer graphene,we realize Q switching mode locking at the average power of 1.5W and with the repetition frequency of 413.3 KHz and pulse duration of 605ns.