【摘要】 报道了一种基于Cr:ZnS多晶材料的克尔透镜锁模激光振荡器。该振荡器采用国产Cr:ZnS多晶材料作为增益介质,成功实现了克尔透镜飞秒锁模运转。其输出光谱呈现独特的马鞍形特征。在5.85 W的泵浦功率下,该振荡器能够稳定输出光谱半峰全宽(FWHM)为211.9 nm、平均功率为1.44 W、脉冲宽度为46 fs的锁模脉冲。这种具有马鞍形输出光谱的激光,可作为基于脉内自差频产生宽光谱中红外激光的理想驱动源,以及大能量中红外飞秒激光放大器的优质种子源,在非线性频率变换与超快激光放大领域中具有重要的应用价值。更多还原

【Abstract】 Objective High-power 2-3 μm mid-infrared（MIR） femtosecond lasers have emerged as indispensable tools in advanced scientific and industrial applications,including time-resolved spectroscopy,environmental trace-gas detection,and lab el-free biomedical imaging,owing to their unique operation in the molecular fingerprint region.Traditionally,such laser sources are generated through optical parametric oscillation（OPO） or difference-frequency generation（DFG） techniques.However,these approaches suffer from several intrinsic limitations,such as relatively low conversion efficiency,complex system architecture,and stringent phase-matching requirements.To achieve higher output power levels,further amplification of the oscillator output becomes necessary,but this introduces a critical challenge:the finite gain bandwidth of the amplifier medium leads to the significant gain-narrowing effect,which not only broadens the pulse duration but also degrades the overall system performance.While spectral pre-shaping techniques can partially address this issue by carefully tailoring the seed spectrum into a saddle-shaped profile prior to amplification,which mitigates gain saturation near the central wavelength,these methods inevitably increase optical complexity and incur substantial energy losses.Therefore,the development of a high-power MIR femtosecond oscillator capable of directly generating a saddle-shaped spectrum represents a significant breakthrough in this field.Methods In this work,we demonstrate a Kerr-lens mode-locked oscillator based on polycrystalline Cr:ZnS through optimizing cavity design and intracavity dispersion control.A 1908 nm fiber laser serves as the pump source,delivering up to 25 W output power with a 6.0 mm beam diameter and a 0.6 mrad divergence angle.The pump beam is focused into the poly crystalline Cr:ZnS gain medium(dimensions of 2 mm ×2 mm × 7 mm;Cr²⁺doping concentration of 4×10^-19 cm^-3) using a 100 mm focal-length plano-convex lens.The crystal is wrapped in indium foil and mounted in a copper holder with active temperature stabilization,maintained at 18 ℃via a semiconductor thermoelectric cooler（TEC） coupled to a water-cooled copper block heat sink.The femtosecond oscillator employs an asymmetric X-folded cavity with a total length of 2.113 m,comprising two curved high reflectors（R1 and R2,radius of100 mm,high transmittance@1.6-1.9 μm,high reflection@2.0-2.7 μm）,a flat-end high reflector（M1,high reflection@2.0-2.7 μm）,and a flat output coupler on a CaF₂ substrate（OC,partial reflectivity of 50%@2.0-2.7 μm）.Using standard ABCD matrix analysis,the beam waist radius inside the crystal is estimated at 32.5 μm,while the pump beam waist radius is measured at 30 μm,ensuring optimized mode overlap for soft-aperture Kerr-lens mode-locking.The end mirror（TM1） is mounted on a translation stage,and modelocked operation is initiated by manually displacing TM1 to induce cavity perturbation.Dispersion compensation is achieved through three chirped mirrors（CM1-CM3,providing—250 fs2 dispersion compensation）and two third-order dispersion mirrors（TM1-TM2,delivering—3000 fs3 dispersion compensation）.This configuration enables precise broadband group delay dispersion（GDD） compensation across the 2.1-2.6 μm spectral range,which is crucial for maintaining stable femtosecond pulse oscillations.Results and Discussions At the incident pump power of 5.85 W,Kerr-lens mode-locking is achieved with an output power of1.44 W,corresponding to an optical-to-optical conversion efficiency of 24.6%.The mode-locked spectrum spans 2.1-2.6 μm.Pulse characterization is performed via second-harmonic-generation frequency-resolved optical gating（SHG-FROG）.The FROG trace displays normalized intensity,while the retrieved pulse profile exhibits a normalized intensity inversion error below 0.004.The reconstructed temporal intensity yields a pulse duration of 46 fs.The retrieved spectral intensity closely matches the experimental spectrum,validating the pulse retrieval fidelity.By combining the pulse repetition rate,average power,output coupler transmittance,and laser beam waist radius,the calculated B-integral within the cavity is 4.97.This substantial nonlinear phase shift indicates strong Kerr nonlinearity,where self-phase modulation（SPM） continuously generates new spectral components.The resulting spectral reshaping suppresses the central intensity while amplifying the sidebands,ultimately forming a saddle-shaped spectrum.The spectrum shows a fundamental frequency signal-to-noise ratio（SNR） exceeding 60 dB at 70.99 MHz.Beam quality factors are measured as M_x²=1.04 and M_y²=1.12.Power stability monitoring over 1 h reveals an average output of 1.44 W and the root mean square value of the average power instability of 0.27%,confirming robust long-term operation.Conclusions We demonstrate a Kerr-lens mode-locked laser based on polycrystalline Cr:ZnS,achieving stable femtosecond operation with a saddle-shaped spectrum.At 5.85 W pump power,the oscillator delivers a 46 fs pulse with a 211.9 nm bandwidth and a 1.44 W average power.This distinctive spectral profile makes the source particularly valuable for broadband mid-infrared generation and ultrashort pulse amplification technologies.更多还原