中红外Fe∶ZnSe连续及温度调谐激光性能研究

Performance of Mid⁃Infrared Fe∶ZnSe Continuous and Temperature⁃Tuned Lasers

宽带可调谐中红外光源在生物医疗、环境监测等众多领域中有着广阔的应用前景。报道了利用自制的Er∶Y_(2)O_(3)陶瓷激光器端面泵浦Fe∶ZnSe晶体,实现连续及基于温度调控的3~5μm波段中红外调谐激光输出。在103 K低温制冷条件下,获得了平均功率为352 m W、中心波长为4170 nm的连续中红外激光。进一步研究了基于温度调控的Fe∶ZnSe激光器的光谱调谐性能,成功实现了383 nm(4170~4553 nm)带宽的连续调谐中红外激光输出。

Objective Continuous and wide-tunable laser sources in the mid-infrared range of 3-5μm have attracted significant attention in fields such as spectral analysis,remote sensing,medical treatment,environmental monitoring,and optoelectronic countermeasures.Currently,the primary approach to achieve broadly tunable lasers in this wavelength range is nonlinear frequency conversion methods such as those using optical parametric oscillators(OPOs)and optical parametric amplifiers(OPAs).These methods have advantages,such as a wide tuning range and technological maturity.However,they typically require high-performance near-infrared pulsed lasers or narrow-linewidth lasers as pump sources,leading to challenges such as an extensive system volume,relatively complex resonators,low optical conversion efficiency,and high costs.In recent years,increasing attention has been paid to schemes that directly generate tunable mid-infrared lasers,including semiconductor quantum cascade lasers and mid-infrared oscillators based on mid-infrared laser gain media.Among them,the Fe∶ZnSe crystal exhibits broad absorption and emission spectra,large absorption and emission cross-sections,as well as low phonon energy,making it one of the best candidate materials for directly generating broadly tunable mid-infrared laser sources in the 3-5μm spectral range.Since the first Fe∶ZnSe mid-infrared laser was reported by Adams et al.in 1999,many studies have been conducted on Fe∶ZnSe lasers.However,stable continuous-wave(CW)and wavelength-tunable Fe∶ZnSe lasers have seldom been domestically reported beyond 4μm in the mid-infrared wavelength.In this study,pumped by a homemade Er∶Y_(2)O_(3)ceramic laser,CW and wideband tunable Fe∶ZnSe lasers are demonstrated in the 3-5μm spectral range.Methods According to the Füchtbauer-Ladenburg(F-L)equation,the emission cross-section of a Fe∶ZnSe crystal is influenced by its fluorescence spectrum and spontaneous emission lifetime.Moreover,it has been demonstrated that there is a noticeable redshift in the central wavelength of the fluorescence spectrum of the Fe∶ZnSe crystals with increasing temperature.Therefore,the emission cross section of the Fe∶ZnSe crystals is temperature-dependent.By controlling the operating temperature of the Fe∶ZnSe crystals,it is possible to achieve a temperature-induced gain spectrum shift,enabling a wavelength-tunable output of the Fe∶ZnSe laser.In the experiment,the Er∶Y_(2)O_(3)ceramic gain medium has a length of 10 mm and a diameter of 1 mm.The atomic fraction of doped Er^(3+)of the sample is 7%.The two end faces of the ceramic are laser-grade-polished and plated with antireflection films in the 3μm wavelength band.A compact two-mirror plano-plano resonator is employed for the laser oscillation.The pumping source is a fiber-coupled semiconductor laser with a maximum output power of 100 W centered at 976 nm.The gain medium is mounted on a heat sink and directly water-cooled to remove the heat accumulated during pumping.The temperature of cooling water is maintained at 15℃.An output power of 3.77 W with a central wavelength of 2740 nm is obtained using an Er∶Y_(2)O_(3)ceramic laser.Then,the 2740 nm laser is collimated by a convex lens F3.After being reflected by the two flat mirrors,M2 and M3,it is further focused on the Fe∶ZnSe laser resonator as the pump light through a lens F4.The Fe∶ZnSe crystal used in the experiment is 6.5 mm long and has a cross-section of 3 mm×3 mm,with an Fe^(2+)concentration of approximately 5×10^(18)cm^(-3).The Fe∶ZnSe laser resonator consists of a plano-concave input mirror(M4)with a curvature radius of 100 mm and a plano-plano output mirror(OC2).The output mirror has a transmissivity of approximately 5%in the 4-5μm wavelength range.The total length of the laser resonator is approximately 68 mm.Based on the ABCD matrix,the laser beam waist radius at the Fe∶ZnSe crystal position is calculated to be~258μm.To achieve an effective CW laser operation,it is necessary to cool the Fe∶ZnSe crystal to ensure a sufficiently long upper-level lifetime.In this study,a low-temperature vacuum chamber cooled with liquid nitrogen is designed.The Fe∶ZnSe crystals are wrapped in an indium foil and mounted on a copper heat sink.The copper heat sink is installed on a Dewar inside the vacuum chamber.The vacuum chamber is equipped with CaF_(2)window plates on both sides.These window plates are coated with broadband antireflection coatings in the mid-infrared range,ensuring a transmissivity of over 96%for both pump and laser wavelengths.Results and Discussions First,the CW laser characteristics of the Fe∶ZnSe crystal are studied under liquid-nitrogen cooling at 103 K.In the experiment,the maximum output power of the Er∶Y_(2)O_(3)ceramic laser is set to 3.0 W to ensure stable operation of the pumping source.The corresponding pump power incident on the Fe∶ZnSe crystal is approximately 2.17 W owing to the Fresnel reflection losses and absorption of the optical components.The output power of the Fe∶ZnSe laser is measured using a thermal sensor power meter.The laser threshold for the incident pump power is approximately 231 mW.The laser output power exhibites an approximately linear increase with the incident pump power.When the pump power reaches 2.17 W,a CW laser output of 352 mW is obtained,and the slope efficiency of the incident pump power is approximately 19.2%.The laser spectra are measured using a mid-infrared spectrometer.It exhibits a single-peak structure with a central wavelength of 4.17μm and a spectral bandwidth of approximately 35 nm.Subsequently,the wavelength tuning performance modulated by the temperature of the Fe∶ZnSe laser is investigated.As the temperature of the Fe∶ZnSe crystal increases,the central wavelength of the laser shifts from 4170 nm at 103 K to 4553 nm at 173 K,resulting in a tuning range of 383 nm.In the experiment,the laser successfully achieves a wide spectral tuning range.When the temperature of the Fe∶ZnSe crystal exceeds 173 K,the upper-level lifetime of the Fe∶ZnSe crystal quickly decreases.Therefore,longer wavelengths are not used in this experiment.Conclusions By employing a homemade Er∶Y_(2)O_(3)ceramic laser as a pump source,stable CW and wideband tunable Fe∶ZnSe lasers are demonstrated.An average output power of 352 mW is obtained at 4170 nm under liquid-nitrogen cooling to 103 K.Furthermore,the wavelength-tuning performance of the Fe∶ZnSe laser modulated by temperature is investigated,and a continuous tuning bandwidth of 383 nm(4170-4553 nm)is successfully achieved in the experiment.