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长波红外量子级联激光器的高效率光纤合束

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长波红外量子级联激光器(QCL)具有波长设计灵活、体积小、寿命长等优点。目前单横模QCL较低的输出功率(1~3W)是限制其应用的主要因素。光纤功率合束技术是提升输出功率的有效手段。然而由于长波红外波段缺少低传输损耗的玻璃光纤,使得高效率长波红外光纤功率合束的实现难度很大。本文研究了基于低损耗单模空芯光纤的长波红外激光功率合束技术。针对基横模长波红外QCL有源区尺寸大、发散角大的特点,设计了大数值孔径扩展光源双非球面准直镜,有效提高了单模光纤耦合效率。设计制备了无端面损耗的长波红外单模光纤束,光纤传输效率高达91。2%,实现了 7。6~7。8 μm波段QCL的高效率合束。当4个长波红外QCL的输出总功率为2。27 W时,采用所设计的光纤耦合光学系统及制备的4×1单模空芯光纤合束器获得了 1。5 W的连续输出,总合束效率为66%。此外,测量得到单根单模长波红外光纤耦合输出光的光束质量因子M2为1。2,光强分布和光束质量因子均优于QCL的直接输出激光,说明空芯单模光纤具有一定的非高斯光束模式净化作用。合束光束的传输质量因子为2。6,依然具有较好的光束质量。本文所研究的光纤合束方式对QCL的输出波长、偏振态均不敏感,且具有良好的可扩展性。实验结果表明,此方式可有效解决长波红外QCL单元器件输出功率偏低的问题。
High-Efficiency Fiber Combining of Long-Wave Infrared Quantum Cascade Lasers
Objective Quantum cascade laser(QCL)is a semiconductor laser based on sub-band electronic transition,which results in a broad emitting wavelength covering from 3 to 300 μm.QCL is an ideal light source in the fields of gas sensing,environmental monitoring,medical diagnosis,and photoelectric countermeasures.However,the relatively low output power(1-3 W)of the single transverse mode QCL is a major limitation for its applications.Laser beam combining technology is an effective way to enhance the output power.At present,the power beam combining of mid-infrared and long-wave infrared QCLs is heavily limited by the low-loss optical materials and component preparations.Beam combining with high efficiency and low loss is challenging,and few research results have been reported.Therefore,the fiber combining of long-wave infrared QCLs in the 7.6-7.8 μm wavelength band was studied in this paper.The laser power was combined with a 4-in-1 single-mode hollow-core fiber bundle.Methods In order to realize the high-efficiency single-mode fiber coupling of QCLs,the optical fiber coupling system was designed.The optical fiber system was composed of a QCL collimator and a fiber coupler.Due to the large QCL emitting area and large divergence angle,an aspheric collimator with a large numerical aperture was designed and fabricated.During the optical design and optimization,the QCL was assumed to be an extended light source.Using the optimized collimator,a fiber coupling efficiency of 88.9%was obtained.To combine the laser beams from individual QCL,a 4-in-l fiber combiner was fabricated using AgI/Ag single-mode hollow-core fiber,which had a high damage threshold and low transmission loss.During the preparation,the outer protective layer of the fiber was stripped away,and the four fibers were tightly arranged in the sleeve and fixed.Finally,the fiber was protected by metal armor.The input terminals of the fiber combiner were four independent SMA905 fiber connectors,and a unified SMA905 connector was made at the output end.Results and Discussions The optical fiber coupling experiments are conducted using the designed optical fiber coupling system and the prepared long-wave single-mode hollow-core fiber combiner.When the QCL output power is 642 mW,the laser power throughout the single-mode fiber is 438 mW.The corresponding fiber coupling efficiency is 68%.In addition,we experimentally compare the coupling efficiency using a point-source collimator and an extended-source collimator.Using the extended-source collimator with a large numerical aperture,the fiber coupling efficiency is increased from 59%to 68%,as shown in Fig.10.An infrared camera is used to observe the collimated QCL spot and the beam spot out of the single-mode fiber.In addition,the beam propagation quality factor M2 after the fiber coupling is calculated.After the fiber coupling,a symmetric Gaussian distribution is observed,and the beam quality is improved to 1.2,compared to the M2 in Table 7.On the basis of the single-channel optical fiber coupling experiment,the optical fiber combining experimental setup of a four-channel long-wave infrared QCL is built.When the total input power from four QCLs is 2.27 W,the fiber combining power is 1.5 W.The corresponding combining efficiency is 66%.In order to evaluate the beam quality of the combined beam,the lens is used to focus the output light,and the intensity distribution of the output spot of the beam combiner is measured within two times Rayleigh distance.The results are shown in Figs.14 and 15.The transmission quality factors of the combined beam are calculated as M2X=2.67 and M2Y=2.56,which meant a good beam quality.Conclusions In this paper,the long-wave infrared QCL beam combining technology based on single-mode hollow-core fiber is studied.Considering the large emitting area and big divergent angle of the fundamental transverse mode long-wave infrared QCL,a QCL collimator with a large numerical aperture is used.During the design,the QCL is treated as an extended light source.To obtain the optimized collimation result,both surfaces of the collimator are aspheric.A 4-in-1 fiber combiner is fabricated using the AgI/Ag single-mode hollow-core fiber.The fiber has no end face reflection loss and low transmission loss.The experimental results show that the single-mode fiber coupling efficiency is 68%.After the fiber coupling,the beam propagation quality factor M2 is 1.2.In addition,the power combining of four QCLs in the wavelength band of 7.6-7.8 μm is realized.When the input power is 2.27 W,the combined output power is 1.5 W.The beam combining efficiency is 66%.The transmission quality factors of the combined beams are M2=2.67 and M2Y=2.56.The low-loss working band of the fiber combiner ranges from 7 to 15.5 μm.The output optical power can be further increased by increasing the number of QCLs in the beam combining,which provides an effective way to expand the output power and wavelength range in the long-wave infrared wavelength band.

long-wave infraredquantum cascade laserfiber combiningcoupling efficiencyhollow-core fiber

张梦、王欣、杨苏辉、李宝、李卓、张金英、高彦泽

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北京理工大学光电学院,北京 100081

北京理工大学学精密光电测试仪器及技术北京市重点实验室,北京 100081

北京理工大学信息光子技术工业和信息化部重点实验室,北京 100081

中国电子科技集团第十一研究所,北京 100015

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长波红外 量子级联激光器 光纤合束 耦合效率 空芯光纤

2024

光学学报
中国光学学会 中国科学院上海光学精密机械研究所

光学学报

CSTPCD北大核心
影响因子:1.931
ISSN:0253-2239
年,卷(期):2024.44(8)
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