Objective Space-based Doppler wind light detection and ranging technology is a highly competitive field among the world's leading aerospace powers.As a key component of lidar,the performance of the single-frequency pulsed laser source determines the measurement accuracy and detection capability of the entire system.For coherent Doppler wind lidar,a laser pulse width exceeding 100 ns is required to ensure the accuracy of the wind measurement.Moreover,spaceborne lidar systems place stringent demands on laser sources in terms of reliability,volume,and weight.Due to its advanced maturity,efficiency,and reliability,the neodymium-doped 1.06-μm laser finds extensive applications in space.Thus,this study proposes a single-frequency,high-energy 1.06-μm laser with a pulse width of hundreds of nanoseconds,aiming to offer a technical approach for the space-borne coherent detection lidar laser source.Methods A fiber-bulk hybrid amplification system is designed,consisting of a cascaded fiber pre-amplifier chain(Fig.1)and multi-stage solid-state amplifier chain(Fig.2).For the fiber pre-amplifier chain,a distributed feedback(DFB)semiconductor laser with a linewidth of approximately 2 MHz serves as a single-frequency continuous-wave(CW)seeder.A Lorentzian pulse waveform is adopted as the modulation signal for an acoustic-optical modulator(AOM)to chop and reshape the CW seeder into a Lorentzian pulse sequence at a repetition frequency of 60 Hz and pulse width of approximately 149.0 ns.The obtained pulsed seeder is then coupled to a Yb-doped single-mode fiber(YSF)amplifier to extract energy and is further amplified by a Yb-doped double-clad fiber amplifier(YDF).To enhance the signal-to-noise ratio,another AOM is utilized with a square modulation signal before the YDF amplifier.A fiber end cap is used at the output of the YDF amplifier to reduce the optical power density at the fiber facet,and the output is collimated using a collimator with a focal length of 4.6 mm,which enters the subsequent solid-state amplification system for further pulse energy scaling.The solid-state amplification system is developed using a fiber-coupled laser diode(LD)end-pumped Nd∶YVO,crystal,acting as a high-gain double-pass preamplifier,followed by an LD array single-side-pumped Nd∶YAG slab preamplifier with a double-pass configuration.Finally,a two-stage LD array double-side-pumped Nd∶YAG slab serves as the power amplifier.For the Nd∶YVO4 crystal preamplifier,a double pass is achieved through angular displacement due to the polarization dependency of the vanadate crystal.After the first pass,the amplified beam is returned with approximately a 3°angular change of the beam direction via the dichromic mirror M2,which is coated with 0°anti-reflection(AR)films at 808 nm and high reflectivity(HR)films at 1064 nm.The Nd∶YAG preamplifier has a zigzag pass with Brewster angle faces,and a double pass is achieved by polarization rotation using a Porro prism and 0.57°plate.The two power amplifiers are single-pass and pumped onto the zigzag total internal reflection point.The Nd∶YAG slabs are conductively cooled from top to bottom by making contact with a conductively cooled Cu heat sink.The first slab power amplifier is cut at the Brewster angle,while the second is cut at an angle of 40°,and also has a near-normal incident.Results and Discussions The modulation signals for AOM1 and AOM2(Fig.3)are Lorentzian waveforms with a pulse width of approximately149.0 ns and rectangular waveforms with a pulse width of 2.4μs,respectively.With pumping at a 1.2 ms pulse width and peak power of 580 mW for LD1 and 525 mW for LD2,the fiber amplifier produces 2.1 μJ pulse energy with a 216.7 ns pulse width(Fig.4).After collimation,the measured diameter of the near-field spot is approximately 0.6 mm,and the divergence is approximately 2.7 mrad(Fig.5).To further scale the pulse energy,the output of the fiber amplifier undergoes amplification using a multi-stage solid-state amplification system.The maximum pulse energy of 151.4 mJ is successfully achieved,with an optical-to-optical efficiency of approximately 7.3％relative to the total incident pump energy.The pulse width of the second slab amplifier output is approximately 267.8 ns with a rising edge of 191.7 ns and a falling edge of 161.4 ns(Fig.6).The measured laser beam quality factor(M2)is 1.39 and 1.60,respectively,in the x direction and y direction at a pulse energy of 151.5 mJ with a laser beam quality analyzer(Fig.7).The inset of Fig.7 displays the near-field intensity distribution of the laser beam.Using a laser wavelength meter,the center wavelength of the pulse laser measures at 1064.49 nm,and the obtained linewidth of less than 500 fm is limited by the laser wavelength meter itself.To achieve an accurate linewidth,a self-built real-time monitoring system for the laser spectrum is employed.Based on optical heterodyne,the center frequency and linewidth of the laser pulse can be calculated according to the beat signal of the laser pulse and reference CW seeder.The linewidth stability of about 1.7×105 pulses is determined(Fig.8),and the mean value of the linewidth is approximately 14.2 MHz with a stability of about 0.25 MHz(root mean square).By adjusting the pulse width of the Lorentz modulation signal of the AOM1(Table 1),the study on the amplified pulse waveform reveals that the laser output achieves a pulse width in the range of several hundred nanoseconds,thus meeting the specific pulse width requirement of coherent detection lidar.Conclusions A hundred-nanosecond,single-frequency,high-energy 1064 nm laser based on fiber-bulk hybrid amplification undergoes experimental investigation as a laser source for space-based coherent detection wind lidar.A Lorentzian pulse waveform reshapes and chops the output of the CW DFB laser.A pulsed seeder with a pulse width of approximately 149.0 ns and a repetition rate of 60 Hz emerges.After the amplification of the cascaded fiber amplifier and multi-stage solid-state crystal amplifier,the system produces a laser output with a single pulse energy of about 151.4 mJ and a pulse width of about 267.8 ns.Utilizing the optical heterodyne method,the laser linewidth measures approximately 14.2 MHz.By altering the pulse waveform of the Lorentz modulation signal,the pulse width of the output laser can vary within several hundred nanoseconds.The study results offer a new technical route for employing a 1.06 μm laser source for space-based coherent detection wind lidars.
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