Study On High Peak Power Pulsed Fiber Amplifiers Based On Synchronous Pulse Pumping

Pulsed fiber amplifiers have wide applications in industrial processing,national defense research,aerospace and so on,due to excellent beam quality,compact configuration,robust performance and low maintenance cost.The all-fiber pulse laser with high peak power as laser source applies in laser processing,laser cleaning,laser radar and laser ultrasonic detection as a consequence of the adjustable repetition frequency and pulse width.However,the output power scales are severely limited by amplified spontaneous emission(ASE)in pulsed fiber amplifiers with low repetition frequency.At present,there are a variety of methods to suppress ASE,in which filtering and synchronous pulse pumping are major technical means to suppress ASE and achieve high-peak-power high-energy pulse outputs.The filtering methods are mainly used in the preamplifier with low power,and synchronous pulse pumping is one of the most potential ways to suppress ASE which adopts the targeted pumping to reduce the ASE between the seed pulse interval in theoretical.Based on the above,we studied the ASE suppression effect in the pulse fiber amplifiers with synchronous pulse pumping at tens of kHz repetition frequency.The main contents are as follows:The application and current research of pulsed fiber amplifiers are introduced,and ASE is the main limiting factor of power boost in pulsed fiber amplifiers with low repetition frequency.We introduce the current research of various methods in ASE suppression.At the repetition frequency of tens of kHz,filtering is the main technical way to filter ASE in low power,and synchronous pulse pumping is important technical means to suppress ASE and obtain high-peak-power high-energy pulse outputs.The generation mechanism of ASE is analyzed,and the amplified feature of ASE in pulsed fiber amplifiers with main oscillation power amplification(MOPA)configuration is studied.We studied the basic principle of suppressing ASE by synchronous pulse pumping theoretically,and analyzed the advantages of synchronous pulse pumping at low repetition frequency compared with continuous pumping.The theoretical model of synchronous pulse pumping is established.Based on the dynamic rate equation of pulsed fiber amplifiers,a parallelizable bidirectional algorithm which can be used to solve the partial differential equation is recommended.We analysed the ASE characteristics of the fiber amplifiers and the energy storage characteristics of the gain fiber through simulation,and discussed the influence of seed power and pump power on ASE characteristics and the influence of gain fiber parameters on the energy storage characteristics.We carried out the experiment of weak pulse signal amplification with 1064 nm to study the amplification characteristics of the weak pulse signal in the continuous pump and pulse pump.At the repetition frequency of 40 kHz,we implemented continuous pump and pulse pump experiments among the pump pulse width of 1 ~ 6 ?s under the same seed power,and the average pump power is almost the same in the two pumping modes.In the synchronous pulse pumping,the maximum output power is 4.58 mW,and the optical-to-optical conversion efficiency is 3.42%,obtained the gain of 11.85 dB.In the continuous pumping,the maximum output power is 2.068 mW and the optical-to-optical conversion efficiency is 1.41 %,achieved the gain of 8.4 dB.The experimental results show that synchronous pulse pumping achieved more than twice the output power and optical-to-optical conversion efficiency with the same pump average power compared to continuous pumping at the repetitive frequency of 40 kHz.The high peak power pulse fiber amplifier experiment with 1064 nm was carried out via synchronous pulse pump.We established a cascade three-stage fiber amplifier system of MOPA configuration.In the synchronous pulse pumping,the seed pulses of average power 941 ?W,pulse enengy 9.41 nJ and peak power 1.02 W are amplified to average power 8.9 W at the repetition frequency of 100 kHz,and the output pulses of pulse energy 89 ?J,peak power 8.93 kW are achieved.The gain of amplifier is 39.76 dB,and the pulse width extends from 8.7 ns to 9.36 ns.By testing the output characteristics of the first stage amplifier and the second stage amplifier,we studied the output characteristics of the amplifiers with different pump pulse width,different repetition frequency and the same duty cycle.Using synchronous pulse pumping with different pump pulse width,the output performance of amplifiers has not been improved with shorter pump pulse width due to the high repetition frequency and the limitation of pump power.When we adopt synchronous pulse pumping with different repetition frequency,synchronous pulse pumping with lower repetition frequency has not show obvious advantages which is limited by pump power.Under the same duty cycle,when the pump power is enough,amplifiers with lower repetition frequency have better output characteristics.So,reducing the repetition frequency of the amplifier can better reflect the advantages of synchronous pulse pumping.We studied the output spectrum characteristics of the synchronous pulse pumping under the high peak output power,and the experimental results show that the nonlinear effect is the main limiting factor of power boost in pulsed fiber amplifier at the high peak output power.In this paper,we theoretically analysed and experimentally proved that the synchronous pulse pump in the weak pulse signal amplification has the significant advantages of enhancing system efficiency and output power,and reducing the repetition frequency of the amplifier can better reflect the advantages of synchronous pulse pumping.Under the high peak output power,the output spectra extended by self-phase modulation(SPM)surpass ASE and become the important limiting factor of power boost in pulsed fiber amplifier.The theoretical and experimental researches have some guide influences on the power scaling of the pulse fiber amplifiers.