Research On Tandem Pumping Technique For Fiber Lasers At 2?m

High power fiber lasers operating at 2?m have promising prospects in various fields,such as laser communication,eye-safe LIDAR,environmental monitoring,medicine,and material processing.2?m high power fiber laser is also a kind of powerful sources suitable for mid-IR laser generation at 3-5mm.Therefore,the power scaling of 2?m fiber laser has become a worldwide research hotspot.Compared to the state of art of 1mm high power Yb-doped fiber lasers,however,Tm-doped and Ho-dopded fiber lasers at 2?m are now faced with challenges.Due to the slowly-developed laser diodes,the immature fiber devices and moderate efficiency-induced severse heat load,the maximum output power of single-mode fiber laser at 2?m is one order of magnitude lower than that at 1mm.How to break through the bottleneck to further improve the output power has become the research priority of 2?m fiber lasers.In this paper,we focused on a technique,tandem pumping,which would be useful to further increase the power of 2?m fiber lasers.The related theoretical and experimental researches have been done in the Tm-and Ho-doped fiber lasers.We analyzed the development progress of 2?m high power fiber lasers.The progress of multimode fiber-coupled laser diodes for 2?m fiber lasers is slow.The brightness of them is much lower than that for 1mm fiber lasers,which makes it difficult to provide sufficient pump brightness.And the operating spectral range of those high power laser diodes is limited,which result in high quantum defect,low efficiency,high heat load and limited output power in diode-pumped fiber lasers at 2?m.With the advantages of high pump brightness and low quantum defect,tandem pumping technique has the potentail to break through bottlenecks induced by diode-pumping,and to further improve the output power of 2?m Tm-and Ho-doped fiber lasers.We did the theoretical investigation on 2?m tandem pumping technique.The rate equations of the tandem-pumped Tm-doped fiber laser was established,which was applied to analyze the influences of active fiber length and core/cladding ratio to the laser efficiency at different pump wavelengths.With the power scaling model of fiber laser,the power scaling limits of Tm-and Ho-doped fiber lasers under different pumping schemes were calculated.And the optimal pumping schemes were selected based on the calculated results.For Tm-doped fiber laser,the optimal pumping scheme is tandem pumping with pump wavelength of 1900 nm.However,the special design of Tm-doped fiber would be required in practice to achieve efficient pump absorption.For Ho-doped fiber laser,the power limits of tandem pumping with pump wavelengths of1150 nm and 1950 nm were approximated to each other.Between these two pumping schemes,tandem pumping with pump wavelength at 1950 nm has more potential,but the optical loss problem of Ho-doped fiber need to be solved at first.We did experimental research on continuous-wave?CW?Ho-doped fiber laser based on tandem pumping.A home-made Raman fiber laser with central wavelength of1150 nm and the maximum output power of?700 W was constructed in the first place.Benefiting from the high pump brightness of tandem pumping scheme,a CW high power Ho-doped fiber laser was built,employing powerful Raman fiber laser at 1150nm as pump source and a piece of large-mode-area Ho-doped fiber as gain medium.The central wavelength of Ho-doped fiber laser was 2050 nm.The maximum output power was exceeding 100 W level,and the slope efficiency was?24%versus absorbed pump power.As far as we know,this is the highest output power when using 1150 nm pump source and common large-mode-area Ho-doped fiber.We did experimental research on pulsed Tm-doped fiber amplifier based on tandem pumping.A mode-locked fiber laser at 2?m was demonstrated employing black phosphorus nanoplates as the saturable absorber.The self-start operation of mode-locking has been verified for more than 20 days,and none of the obvious changes of output characteristics were observed.The mode-locked fiber laser has a high signal-to-noise ratio.Then we employed a 2?m mode-locked fiber laser as the seed laser,and a CW Tm-doped fiber laser operating at 1.94mm as the pump source,to construct a free-space high-average-power high-efficiency Tm-doped fiber amplifier.With the advantages of low quantum defect and low heat load,the maximum average output power of the pulsed Tm-doped fiber amplifier reached 40 W,with the pulse duration of 129 ps and the repetition rate of 248 MHz.The slope efficiency of the amplifier reached 87%,which is the highest slope efficiency of the pulsed Tm-doped fiber laser as far as we know.We also explored the application of tandem pumping in special wavelength generation.The related experiements have been done to verify the feasibility.To achieve high power laser emission at>2.1mm,challenges such as small emission cross section and lack of high-brightness pump sources have to be overcame.We proposed three schemes based on tandem pumping technique:?1?Using the Raman gain in highly GeO₂-doped passive fiber,and employing a home-made high-power pulsed Tm-doped fiber laser at 1.94mm as pump laser,we built a random distributed feedback fiber laser at 2119 nm,which was the first demonstration of the random distributed feedback fiber laser at 2.1mm;?2?Employing a 1173 nm high-power random distributed feedback fiber laser as pump source to increase the net gain at long wavelength,a high-power Tm-doped fiber laser at 2153 nm with the output power of 18 W was achieved.The output power was improved by 8 times compared to the 793 nm laser diode-pumped long-wavelength Tm-doped fiber.The ASE and parasitic lasing within the common band of Tm-doped fiber laser were effectively suppressed;?3?Using 100 W-level fiber laser at 1.15mm as pump source,a high power dual-wavelength Ho-doped fiber laser was demonstrated in the coupled-cavity configuration.The central wavelengths were2049 nm and 2153 nm,and the maximum output power reached 22.3 W.The power ratio between two wavelengths could be adjusted by changing the incident pump power.