| Fiber lasers with huge advantages such as low cost,excellent beam quality,good stability,high conversion efficiency and simplicity have always been one of the hot spots in the laser field.Pulsed fiber lasers meet the requirement of applications in many fields including material processing,scientific research and biomedicine based on its superiority such as high peak power,narrow pulse duration and high repetition rate and so on.As the rapid development of pulsed fiber lasers,several kinds of approaches to pulse operation have been exploited and improved.Benefit from the complicated energy lever structure of those rare-earth-doped elements,fiber lasers possess extremely ample output wavelengths,which covers a wide range from ultraviolet to mid-infrared.Among these wavelengths,1.7 ?m falls into the spectral gap between the Er3+ and Tm3+,which leads to an insufficient research and achievement.Because of the special spectral properties,therefore,1.7 ?m laser sources get great application prospects in the fields of laser medical treatment,biological imaging,mid-infrared laser generation,special materials processing and organic matter micro-measurement,drawing extensive concerns in recent years.Although 1.7 ?m continuous wave fiber lasers have already been developed to some extent under the great efforts of researchers,it is a great pity that pulsed fiber lasers operating in 1.7?m have not been fully exploited and investigated.So achieving the 1.7 ?m fiber laser,and then adopting different plans to realize low cost,high performance and all-fiber structure pulsed fiber lasers within 1.7 ?m region is undoubtedly a meaningful research.In this graduate academic paper,the research mainly focuses on the 1.7 ?m pulsed fiber laser,the detailed research includes two aspects:1)1.61?1.85?m tunable Raman soliton source based on soliton self-frequency shift,2)exploiting intermodal-beating modulation mechanism to achieve 1.78 ?m all-fiber pulsed laser in thulium-holmium(Tm/Ho)co-doped fiber.The main results are summarized as follows:1.According to the comparison of conventional single mode fiber with phosphor-doped fiber in Raman spectra,we find that the Si02/Ge02 Raman shift of phosphor-doped fiber is much wider than one of the conventional silica fiber.Therefore,it can be expected the soliton self-frequency shift in the phosphor fiber could extend the wavelength into 1.7 ?m region under a 1.56 ?m pumping.The amplified dissipative soliton pulses at 1.56 ?m is finally injected into the 960 m phosphor-doped fiber,we achieve a 1.61?1.85 ?m tunable all-fiber Raman soliton source based on the soliton self-frequency shift.Although the spectral range is wide enough,unlike the supercontinuum spectrum,our Raman solitons always keep the single-soliton state without pulse-splitting.2.This research group firstly proposed the intermode-beating modulation technique in 2012,and then utilized this novel technique to obtain pulsed laser operation in Raman gain fiber and rare-earth-doped fiber for the first time.This provides guidance for our achievement in 1.7 ?m pulsed fiber laser.To obtain 1.7 ?m laser,we chose Tm/Ho co-doped fiber as the gain medium.Because this fiber has a strong absorption at 1211 nm,we build a 1211 nm Raman fiber laser as the pump source.Eventually we accomplish a 1.78?m Tm/Ho co-doped all-fiber pulsed laser based on inremode-beating modulation technique.In particular,different from the feature of passive Q-switching,the repetition rate of 1.78 ?m pulsed fiber laser is fixed at 145 kHz under different pump power,which exactly coincides with the longitudinal-mode beating frequency of the 1211 nm pump laser.What is more,by adjusting the length of gain medium in cavity,we are able to change the output wavelength of laser,including:1.80 ?m,1.76 ?m and 1.70 ?m,which covers the 1.7 ?m region. |
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