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设计合理的激光器谐振腔腔型,不仅获得较大的激光输出能量,也使得输出激光光束参数满足光纤的耦合条件。激光器谐振腔输出高斯光束远场发散角
$\theta $ 的关系式为[11]:式中:
$n$ 为谐振腔输出镜的折射率;${R_1}$ 和${R_2}$ 分别为输出镜的内外曲率半径;${\;\rho _2}$ 为出射光束等相面曲率半径;${\omega _2}$ 为出射高斯光束半径;$\omega ({\textit{z}})$ 为z处光斑半径;$\lambda $ 为激光波长。从公式(1)和(2)可以看出,谐振腔输出镜的内外曲率半径以及腔镜的折射率对输出高斯光束远场发散角有较大影响。在不考虑传输系统的像差时,从激光器谐振腔出射后的激光光束在传输过程中其光束参数乘积BPP值保持不变,即激光光束束腰半径和发散角成反比例关系,发散角增大时,束腰光斑直径就增大。从激光器谐振腔出射的高斯光束必须满足光纤的耦合条件,才能经过耦合透镜变换后与光纤参数相匹配。光纤耦合条件为激光光束束腰半径与远场发散角乘积的BPP值必须满足[12]:
式中:
${d_{core}}$ 为光纤芯径;$NA$ 为光纤的数值孔径。谐振腔腔型为平凸型,为非稳定腔。通过理论计算前腔镜焦距,但217 mm<R1<412 mm和279 mm<R2<453 mm时,输出光束参数乘积满足ZBLAN 光纤的耦合条件。通过对比不同焦距的透镜作为输出镜时的激光输出功率的大小,能够从中选择最优参数的透镜作为谐振腔输出镜。采用焦距分别为R1=250 mm,R2=300 mm和R1=300 mm,R2=350 mm以及R1=350 mm,R2=400 mm的氟化钙透镜作为耦合透镜,测量它们的输出功率的对比图,如图1所示。
从实验结果可以看出,在进行耦合实验时,在满足光纤耦合的条件上,尽可能地选择使激光器输出功率较大的透镜作为谐振腔的输出镜。最终选择了参数为R1=250 mm,R2=300 mm的弯月形透镜作为谐振腔输出镜,并通过实验测量了不同参数的透镜作为耦合透镜时其输出激光的BPP值,来验证理论计算的准确性。
在激光器输出光束直径一定的情况下,若激光束远场发散角较大,其光束参数乘积不满足光纤耦合条件,就无法耦合进光纤。实验中的ZBLAN光纤数值孔径NA=0.29、光纤芯径为400 μm。由公式(3)计算得到耦合的激光光束参数BPP值需要小于59.2 mm·mrad才能满足光纤耦合条件。从公式(1)、(2)可以看出,采用不同曲率半径的透镜替代平面镜作为谐振腔输出镜,可以改变输出光束的发散角。为此,笔者设计了弯月型输出镜的激光器谐振腔,以减小激光光束发散角。实验中激光器采用Er, Cr:YSGG晶体圆棒的尺寸为Ф4 mm×70 mm,
$ {\rm{Er}}^{3+} $ 的浓度为20at.%,$ {\mathrm{C}\mathrm{r}}^{3+} $ 的浓度为3at.%,晶体棒的两端镀有对2.79 μm的增透膜,后腔镜为平面全反镜,在2.79 μm处反射率>99%,用不同曲率半径的弯月型氟化钙透镜作为输出镜,其折射率$n$ =1.4349,凹面面向腔内,两边镀有增透膜,对2.79 μm波长的激光透过率为T=20%。由于激光从激光器输出后其光束参数乘积是保持不变的,可测量激光器输出端后任意位置处的光束参数乘积来表示激光器输出光束参数。实验采用距离激光输出镜16 cm处作为测量点,测量不同曲率半径的弯月型透镜作为输出镜的激光光束发散角和光斑直径,并计算它们的BPP值如表1所示。Output mirror parameters/mm Divergence angle/mrad Spot diameter/mm BPP value/mm·mrad 1 R1=∞,R2=∞ 44.63 5.91 131.90 2 R1=150,R2=200 31.10 6.03 93.77 3 R1=200,R2=250 27.62 5.37 74.10 4 R1=250,R2=300 20.60 3.26 33.57 Table 1. Divergence angle of the output mirror with different parameters
通过实验数据计算得到第四组激光束的参数乘积BPP值为33.57 mm·mrad,满足光纤耦合条件。因而选用曲率为
${{R}}_{1}$ =250 mm、${{R}}_{2}$ =300 mm的弯月型透镜作为激光器谐振腔的输出镜。
Experimental investigation of 2.79 μm Cr, Er: YSGG laser fiber coupling
doi: 10.3788/IRLA20210236
- Received Date: 2021-04-13
- Rev Recd Date: 2021-06-25
- Publish Date: 2021-11-02
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Key words:
- Cr, Er: YSGG laser /
- single lens /
- resonant cavity /
- fiber /
- space coupling
Abstract: Using the energy transmitting fiber to replace the optical guide arm can greatly improve the flexibility of the medical handle, reduce the complexity of the system and improve the efficiency of laser transmission. A 2.79 μm Er, Cr: YSGG laser and its fiber coupling system were designed and developed. The influence of the output mirror of the laser resonator on the parameters of the output Gaussian beam was analyzed. A meniscus type lens was designed as the output mirror of the laser resonator to reduce the divergence angle of the laser beam, and a suitable coupling single lens was selected to meet the coupling conditions of the ZBLAN glass fiber with a numerical aperture of 0.29 and a core diameter of 400 μm. The experimental results show that when the meniscus type lens is used as the laser output mirror and the focal length of the coupling lens is 20 mm, the coupling efficiency of the laser transmission can reach up to 83%, and the maximum transmission power is 6 W, which meets the clinical application requirements of the laser medical instrument.