Research On High-energy Dual-wavelength Switching Ho:YLF Injection-seeded Laser Technology

Monitoring of greenhouse gas CO₂ concentration has become a hot topic in the world.The active detection technology applied on differential absorption lidar（DIAL）can achieve high-precision detection of CO₂ concentration in the vertical profile,which makes up for the lack of passive remote sensing technology.The laser at 2μm has the characteristics of eye-safe and low atmospheric absorption.More importantly,the 2μm laser after tuning is located in the absorption peak of CO₂.Therefore,the 2μm laser can be used as an effective transmitter of CO₂ DIAL.Two seed beams with narrow linewidth and high frequency stability are switched and injected into the slave laser with long pulse width by the injection-seeded technology of lidar transmitter,and the output laser with the characteristics of seed and slave laser is amplified by the oscillator-amplifier setup.The final dual-wavelength high-energy single-frequency pulsed laser meets the requirements of CO₂ DIAL.Based on the Ho:YLF crystal,a 2μm CO₂ DIAL transmitter,namely a high-energy dual-wavelength switching injection-seeded laser,has been studied in this paper.First of all,this paper compares the technical means of CO₂ concentration detection,points out the advantages of DIAL,and introduces the research progresses of DIAL.Combined with the requirements of lidar for the transmitter and the advantages of 2μm laser,the research progresses of 2μm injection-seeded laser and lidar transmitter are analyzed and summarized,and the research significance of 2μm high-energy dual-wavelength switching Ho:YLF injection-seeded laser is clarified.The P12 CO₂ line（2064.413 nm）is selected as the on-line wavelength near the emission peak of Ho:YLF crystal.Compared with other research work,the absorption of P12 line is stronger.The absorption and emission spectra of the Ho:YLF crystal are introduced theoretically,and the pumping wavelength and output wavelength of the Ho:YLF laser are discussed.The thermal effect of the Ho:YLF crystal is also analyzed.Based on the energy level transition of the Ho:YLF crystal,the simplified quasi two-level rate equations of the continuous-wave（CW）Ho:YLF laser and amplifier are established.The influences of intra-cavity round-trip loss,crystal length,pump waist radius and transmission of output coupler on the laser output characteristics are analyzed numerically.Meanwhile,the influences of the main oscillator power,beam quality factor and pump waist radius on the amplifier output characteristics are also analyzed.At the pulsed operation,the laser energy extraction efficiency and pulse width are analyzed by using the active Q-switching rate equation.In order to obtain the single-longitudinal-mode seed laser with high stability,the ring cavity composed of double corner cubes with highly anti-misalignment characteristics is used to achieve the long-term stable operation of the laser.The wavelength stability is 0.3 pm in 18 min,which indicates that the frequency stability of the seed laser is high.In theories and experiments,the tuning characteristics of etalon and wedge prism device to the output wavelength of on-line seed laser are analyzed.Based on the coarse tuning of etalon,intra-cavity wedge prism device is used for the first time to realize the high-precision continuous tuning of the output wavelength near the P12 CO₂ line.The continuous tuning range is about 5.75 pm and the tuning accuracy is 0.09 pm/V.Moreover,the output wavelength of on-line seed laser can be controlled in a closed loop by using CO₂ absorption cell.The‘M’ring cavity composed of five mirrors is designed according to the ABCD matrix,which is suitable for both Ho:YAG crystal with positive thermal lens effect and Ho:YLF crystal with negative thermal lens effect.The stability of the resonator under different crystal thermal focal lengths is discussed.In the experimrnt,the output characteristics of‘M’ring Ho:YAG and Ho:YLF laser composed of five mirrors at the continuous and pulsed operation are compared.And then,a 0.1-mm-thick YAG etalon is inserted into the cavity to tune the output wavelength of Ho:YLF slave laser near that of the two seed lasers.Finally,the influences of frequency detuning,mode matching and optical isolation between master and slave laser on the injection-seeded laser are analyzed theoretically,which provides theoretical guidance for the subsequent experiments.The output characteristics of dual-wavelength injection-seeded Ho:YLF laser and single-frequency oscillator-amplifier setup are analyzed in the experiment.The maximum single-frequency pulsed energy,pulsed width and linewidth from the amplifier are 24.2 m J,250 ns and 2.81 MHz respectively at the pulse repetition frequency（PRF）of 100 Hz,master oscillator energy of 2.6 m J and the pump power of 33.9 W.In the CO₂ DIAL transmitter based on Ho:YLF crystal,the long pulse width of 250ns can effectively improve the frequency spectrum resolution of the system.The energy stability of Ho:YLF amplifier is approximately 1%,and the standard deviation of center frequency fluctuation is about 0.85 MHz in 30 min.